About once a day it puts out an audible "alert" but it sounds nice, like chimes, not like a warning, more like a lullaby. What is that?

There is no dinger as the other guy suggests . sound comes from the driver door speaker .
The audible warnings alert the driver of a system concern or a critical vehicle condition. The radio generates the audible warnings through the left front speaker. The radio receives audible warning requests via the class 2 serial data line. If the radio receives multiple audible warning requests, the warning with the highest priority sounds first. On vehicles with an audio amplifier, the amplifier instead of the radio generates the audible warnings and receives audible warnings requests via the class 2 serial data line. Either the radio or the audio amplifier is the chime producer. The following lists the audible warning priority and the pulse rate:
Fast rate chime (150 pulses per min)
Medium rate chime (100 pulses per min)
Slow rate chime (50 pulses per min)
Turn signal reminder (will pulse at turn signal flasher rate)

Your best bet ,wear your seatbelt . so the sound goes off . If you do wear the seat belt and the chime sounds ,have a qualified repair shop check it .

check fuses first, cab and bay
if all good
scan the PCM .
or get it scanned.

your best bet , take it to a qualified repair shop. Have it checked for DTC'S - diagnostic trouble code's

Audible Warnings Description and Operation
The audible warnings alert the driver of a system concern or a critical vehicle condition. The instrument panel cluster (IPC) generates audible warnings through an internal acoustic speaker. The IPC receives audible warning requests via the serial data circuit. If the IPC receives multiple audible warning requests, the warning with the highest priority sounds first. The following lists the audible warning priority and the pulse rate:
Fast rate chime (200 pulses per minute)
Medium rate chime (150 pulses per minute)
Slow rate chime (50 pulses per minute)
Single chime
Door Ajar Warning
The IPC activates the door ajar audible warning as requested by the body control module (BCM). The BCM sends a serial data message to the IPC indicating the chime frequency and duration. The door ajar warning sounds and the appropriate DOOR AJAR indicator illuminates in the driver information center (DIC) when the following occurs:
• The body control module (BCM) determines that a door, driver door, passenger door. The IPC receives a serial data message from the BCM indicating the door ajar status.
• The vehicle is not in park. The IPC receives a serial data message from the BCM indicating the gear position.
• The vehicle speed is greater than 8.05 km/h (5 MPH). The IPC uses the vehicle speed information from the ECM to calculate the vehicle speed.
Fasten Safety Belt Warning
The IPC activates the fasten safety belt audible warning as requested by the BCM. The BCM sends a serial data message to the IPC indicating the chime frequency and duration. The fasten safety belt warning sounds and the fasten safety belt indicator illuminates when the following occurs:
• The ignition switch transitions to ON.
• The inflatable restraint sensing and diagnostic module (SDM) detects that the driver's seat belt is not buckled. The IPC receives a serial data message from the SDM indicating the drivers seat belt status.
If the seat belt is buckled when the ignition is turned ON, the chime will not sound. If the seat belt is buckled while the chime is sounding, the chime will stop. If the seat belt is unbuckled after the initial transition to ON, the chime will not sound.
Key-In-Ignition Warning
The IPC activates the key-in-ignition audible warning as requested by the BCM. The BCM sends a serial data message to the IPC indicating the chime frequency and duration. The key-in-ignition warning sounds when the following occurs:
• The ignition switch is OFF.
• The body control module (BCM) determines that the driver's door is open. The IPC receives a serial data message from the BCM indicating the door ajar status.
• The BCM determines that the key-in-ignition switch is closed and the signal circuit is low. The IPC receives a serial data message from the BCM indicating the key-in-ignition status.
Lights On Warning
The IPC activates the lights on warning as requested by the body control module (BCM). The BCM sends a serial data message to the IPC indicating the chime frequency and duration. The lights on warning sounds when the following occurs:
• The ignition is OFF.
• The BCM determines that the drivers door is open and the signal circuit is low.
• The BCM determines that the headlamp switch is in the park or head position.
Park Brake Warning
The IPC activates the park brake audible warning as requested by the BCM. The BCM sends a serial data message to the IPC indicating the chime frequency and duration.
• The body control module (BCM) determines that the park brake is applied.
• The vehicle is not in park. The IPC receives a serial data message from the engine control module (ECM) indicating the gear position.
• The vehicle speed is greater than 8.05 km/h (5 mph). The IPC uses the vehicle speed signal information from the engine control module (ECM) in order to calculate the vehicle speed.
Additional Warnings
The following warnings have an associated instrument panel cluster (IPC) indicator or driver information center (DIC) message:
• Check Oil Level
The IPC activates the audible warning as requested by the BCM. The BCM sends a serial data message to the chime producer indicating the chime frequency and duration.
• Low Fuel
The IPC activates the audible warning as requested by the BCM. The BCM sends a serial data message to the chime producer indicating the chime frequency and duration.
• Oil Pressure Low Stop Engine
The IPC activates the audible warning as requested by the BCM. The BCM sends a serial data message to the chime producer indicating the chime frequency and duration.
• Reduced Engine Power
The IPC activates the audible warning as requested by the BCM. The BCM sends a serial data message to the chime producer indicating the chime frequency and duration.
• Trans Hot Idle Engine
The IPC activates the audible warning as requested by the BCM. The BCM sends a serial data message to the chime producer indicating the chime frequency and duration.
• Turn Signal On
The IPC activates the audible warning as requested by the BCM. The BCM sends a serial data message to the chime producer indicating the chime frequency and duration

What is it chiming ? Chimes for different reasons , headlamps on , seat belt not buckled , etc.... What warning light is lit on the instrument cluster ? if any . seat belt , air bag's etc...

Audible Warnings Description and Operation
The audible warnings alert the driver of a system concern or a critical vehicle condition. The following lists the audible warning priority and the pulse rate:
Fast rate chime (200 pulses per minute)
Medium rate chime (150 pulses per minute)
Slow rate chime (50 pulses per minute)
Single chime
Fasten Safety Belt Warning
The inflatable restraint sensing and diagnostic module (SDM) activates the fasten safety belt audible warning for 5-8 seconds. The fasten safety belt warning sounds and the fasten seat belt indicator illuminates when the following occurs:
• The ignition switch is in RUN, BULB TEST, or START position.
• The SDM detects that the driver's seat belt is not buckled (signal is low).
If the seat belt is buckled before the ignition is in RUN, BULB TEST, or START position, the chime does not sound. If the seat belt is buckled while the chime is sounding, the chime stops.
Key-In-Ignition Warning
The instrument panel cluster (IPC) activates the key-in-ignition audible warning. The key-in-ignition warning sounds when the following occurs:
• The ignition switch is in OFF, LOCK, or ACCY position.
• The body control module (BCM) determines that the driver's door is open (signal is low).
• The BCM determines that the key-in-ignition switch is closed (signal is low).
Lights On Warning
The instrument panel cluster (IPC) activates the lights on audible warning. The lights on warning sounds when the following occurs:
• The key is not in the ignition.
• The body control module (BCM) determines that the drivers door is open (signal is low).
• The BCM determines that the headlamp switch is in the park or head position.
Rear Parking Assist
The rear object sensor control module activates the audible warning. The audible warning sounds whenever any of the indicators are illuminated on the rear parking assist display. For a description of the rear parking assist system refer to Object Detection Description and Operation.
Turn Signal Reminder
The instrument panel cluster (IPC) sounds the turn signal reminder once per each flash of the turn signal lamps when:
• The ignition switch is in the RUN position.
• The turn signal switch is in either turn position, sensing, within 1 second, an open to B+ transition in the turn signal control circuit (left or right turn).
• The vehicle has traveled a distance of 1.2 km (0.75 mi) by counting pulses on the vehicle speed input circuit.
The IPC turns off the turn signal reminder when either the ignition switch is turned to the LOCK position or to the OFF position, or the turn signal switch returns to the OFF position, sensing, within 1 second, no transitions from open to B+ in either of the turn signal control circuits (left turn or right turn).
Additional Chimes
The following chimes will turn on when an associated indicator turns on in the instrument panel cluster (IPC):
• Low Fuel Indicator
• Low Oil Pressure Indicator
• OVER SPEED indicator
For a description of an indicator refer to Indicator/Warning Message Description and Operation.
For diagnosis of an indicator refer to Symptoms - Instrument Panel, Gages and Console.

Your have a problem with one of these systems , your best bet , take it to a qualified repair shop. They would do diagnostic's to find the problem .

Do any indicators illuminate when the chime sounds?
Go to Symptoms - Instrument Panel, Gages and Console
Go to Step 3
3
Turn the ignition OFF.
Remove the key from the ignition.
Turn the headlamp switch to OFF.
Close all of the vehicle doors.
Open one or more of the vehicle doors
Does the chime sound at all times regardless if the doors are open or closed?
Go to Step 15
Go to Step 4
4
Close all of the vehicle doors.
With a scan tool, observe the LF Door Ajar Switch parameter and the RF Door Ajar Switch parameter in the instrument panel cluster BCM/Inputs data list.
Do all of the Door Ajar Switch parameters display Door Closed?

With a scan tool, observe the Left Rear Door Ajar Sw. parameter and the Right Rear Door Sw. parameter in the instrument panel cluster BCM/Inputs data list.
Do all of the Rear Door Switch parameters display Closed?

This is a 21 step diagnostic trouble shooting flow chart .

Do you still hear warning chimes ?
The audible warnings alert the driver of a system concern or a critical vehicle condition. The radio generates the audible warnings. The BCM receives audible warning requests via the class 2 serial data line or the controller area network (CAN) data line. If the BCM receives multiple audible warning requests, the warning with the highest priority sounds first. The following lists the audible warning priority and the pulse rate:
Do you have base or uplevel system ? Who put in the NEW delco radio ? I suggest you watch this video . GMC Acadia Diagnose Radio Volume Stuck On HIGH Part II A GM radio is the same for chevy , gmc , buick etc... an could need programming , just watch eric .He had radio fixed same problem .Some times you can't get away from going to the dealer.

The audible warnings alert the driver of a system concern or a critical vehicle condition. The radio generates the audible warnings through the speaker. The radio receives audible warning requests via the serial data circuit. If the radio receives multiple audible warning requests, the warning with the highest priority sounds first. The following lists the audible warning priority and the pulse rate:
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Fast rate chime (200 pulses per minute)

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Medium rate chime (150 pulses per minute)

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Slow rate chime (50 pulses per minute)

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Single chime

Are you aware of the fact that your factory radio is connected to the class 2 serial data communication network . You need a interface module to hook up to aftermarket radios . If you don't use this interface module your vehicle will not start an run an the warning chimes will not work .
The audible warnings alert the driver of a system concern or a critical vehicle condition. The radio generates the audible warnings through the left front speaker. The radio receives audible warning requests via the class 2 serial data line. If the radio receives multiple audible warning requests, the warning with the highest priority sounds first. The following lists the audible warning priority and the pulse rate:

Something is not telling the car your door is closed check the light switch make sure it's pushing In and properly and getting contact

The audible warnings alert the driver of a system concern or a critical vehicle condition. The radio generates the audible warnings through the left front speaker. The radio receives audible warning requests via the class 2 serial data line from the IPC or body control module (BCM). If the radio receives multiple audible warning requests, the warning with the highest priority sounds first. The following lists the audible warning priority and the pulse rate:
Fast rate chime--200 pulses per minute
Medium rate chime--150 pulses per minute
Slow rate chime--50 pulses per minute
Single chime

Is the sound heard different from typical Audible Warnings ?

Fasten Safety Belt Warning
The radio activates the fasten safety belt audible warning as requested by the IPC. The IPC sends a class 2 message to the radio indicating the chime frequency, slow rate and duration (5 pulses). The fasten safety belt warning sounds and the fasten safety belt indicator illuminates when the following occurs:
• The ignition switch transitions to ON.
• The inflatable restraint sensing and diagnostic module (SDM) detects that the driver's seat belt is not buckled, signal is low. The IPC receives a class 2 message from the SDM indicating the driver's seat belt status.
If the seat belt is buckled when the ignition is turned on, the chime does not sound. If the seat belt is buckled while the chime is sounding, the chime stops. If the seat belt is unbuckled after the initial transition to ON, the chime does not sound.
Headlamps On Warning
The radio activates the headlamps on warning as requested by the body control module (BCM). The BCM sends a class 2 message to the radio indicating the chime frequency, fast rate and duration, continuous. The headlamps on warning sounds when the following occurs:
• The ignition is OFF.
• The BCM determines that the driver's door is open, signal is low.
• The BCM determines that the headlamp switch is in the park or head position.
Key-In-Ignition Warning
The radio activates the key-in-ignition audible warning as requested by the BCM. The BCM sends a class 2 message to the radio indicating the chime frequency, medium rate and duration, continuous. The key-in-ignition warning sounds when the following occurs:
• The ignition switch is OFF.
• The body control module (BCM) determines that the driver's door is open, signal is low.
• The BCM determines that the key-in-ignition switch is closed, signal is low.
Turn Signal Reminder
The radio activates the turn signal warning as requested by the IPC. The IPC sends a class 2 message to the radio indicating the chime frequency, medium rate and duration, continuous. The turn signal warning sounds when the following occurs:
• The ignition switch is in the RUN position.
• The turn signal switch is in either turn position, sensing, within 1 second, an open to B+ transition in the turn signal control circuit, left or right turn.
• The vehicle has traveled a distance of 1.2 km (¾ mi) by counting pulses on the vehicle speed input circuit.
The IPC turns OFF the turn signal reminder when either the ignition switch is turned to the OFF position, or the turn signal switch returns to the OFF position, sensing, within 1 second, no transitions from open to B+ in either of the turn signal control circuits, left turn or right turn.
Additional Chimes
The following chimes will turn ON when an associated indicator/message turns ON in the IPC/DIC:
• ABS indicator
• AWD Disable indicator
• Battery/Charge indicator
• Brake indicator
• Change Engine Oil indicator
• Door Ajar indicator
• Hot Coolant Temp indicator
• Low Brake Fluid indicator
• Low Fuel indicator
• Low Oil Level indicator
• Low Oil Pressure indicator
• Low Tire Pressure indicator/reset
• Low Washer Fluid indicator
• MIL (Service Engine Soon) indicator
• Oil Life Reset
• Park Lamp Out indicator
• Rear Hatch Ajar indicator
• Service Traction System indicator
• Service Vehicle Soon indicator
• TRAC OFF indicator
For a description of an indicator refer to Indicator/Warning Message Description and Operation .
For diagnosis of an indicator refer to Symptoms - Instrument Panel, Gages and Console .

Did the Mechanic check for DTC'S - diagnostic trouble codes in the BCM or insturment cluster ? Did you try a Buick dealer ?