Does a 12 volt radio/amp use more voltage at higher volume levels

What system does it have ? Base or up level ,does it have a amp ? Do you know what a RPO code is ? Without testing no one could say for sure what the issue is . First off would need to know what system we are dealing with .
RPO Options
The entertainment system on this vehicle is configured with either a base or uplevel audio system. Both the base and uplevel audio systems contain a radio, antenna, and speakers.
The following shows the Enteratinment RPOs that are available for this vehicle:
• (UM7) AM/FM Stereo
• (U1C) AM/FM Stereo CD
• (US8) AM/FM Stereo, CD, MP3, RDS, EQ
• (US9) AM/FM Stereo, 6-Disc CD, MP3, RDS, EQ
• (UXQ4) Base Four Speaker
• (U79) Uplevel Four Speaker
• (WBM) Premium Audio
• (U2K) Digital Radio

Circuit Operation
Radio Power: The main radio power is supplied by the 10A RADIO (BATT1) fuse in the body control module (BCM). Radio switch-on power, 12 volts in ACC, RUN or RAP, is supplied by the 10A RADIO (ACC.) in the BCM.
Radio Grounds: The main radio ground provides a ground for the radio circuits. An additional braided ground strap provides a shielding radio case ground to reduce EMI noise. Both grounds are connected together inside the radio.
Radio Speaker Outputs: At zero volume, the plus (+) and minus (-) speaker outputs are both approximately 5-6 volts, measured to vehicle ground. If a plus or minus for any speaker output is shorted to ground or voltage, the radio circuitry will turn OFF all 4 speaker outputs for component protection. Above zero volume, the plus and minus change to create a voltage difference between each other, to drive the voice coil of the speaker.
Radio Park Lamp Input: The radio park lamp input allows the radio to sense when the vehicle parking lamps are ON. When the parking lamps are ON, this circuit goes to battery voltage, and the radio display back-lighting switches from full bright to the brightness level determined by the I/P illumination input.
Radio I/P Illumination Input: A pulse width modulated (PWM) voltage for instrument panel (I/P) illumination is provided to the radio I/P illumination input. The Radio uses this input to directly illuminate the radio buttons and adjust the radio display back-lighting when the park lamp input is ON.
Amplifier Power: The main amplifier power is provided by the 20A PREM AUDIO fuse in the instrument panel fuse block..
Amplifier Radio Speaker Inputs (WBM only): The radio speaker outputs, at a reduced output level for amplified systems, are the amplifier inputs. The amplifier boosts these inputs and outputs them to the vehicle speakers. Speaker plus and minus circuits from the radio change to create a voltage difference between each other. If one speaker plus or minus is open between the radio and the amplifier, the input to the amplifier is approximately half because only one of the circuits is changing. The speaker for that channel then operates at approximately only half the normal volume.
Amplifier Speaker Outputs (WBM only): At zero volume, the plus (+) and minus (-) speaker outputs are both approximately 5-6 volts, measured to vehicle ground. If a plus or minus for any speaker output is shorted to ground or voltage, the amplifier circuitry will turn OFF either the front outputs or rear outputs for component protection. Above zero volume, the plus and minus change to create a voltage difference between each other, to drive the voice coil of the speaker.
Amplifier Radio-On (WBM only): When the radio is ON, this circuit is pulled to 12 volts by the radio. The amplifier switches ON when this circuit is 12 volts and switches OFF when this signal is 0 volts.
Remote Radio Audio Signal Inputs (UE1 only): Audio output from the OnStar® communications module connects to the remote audio signal inputs of the radio. When the cellular telephone mute signal goes to 0 volts, the radio over-rides any other audio signal and uses these inputs as the source for output to the speakers.
Cellular Telephone Mute (UE1 only): The OnStar® communications module uses the cellular telephone mute signal circuit to over-ride the radio for OnStar® communication. When cellular telephone mute is not active, this circuit is held at 2 volts by the radio. When the cellular telephone mute signal is pulled to ground, the radio over-rides any other audio signal and uses the remote audio signals as the source for output to the speakers. If the radio was OFF when this circuit is pulled low, the radio will turn ON. Additionally, the radio fades the speakers to full front, adjusts the volume to an initial audible level, and sets an AutoTone designed for optimal use with OnStar®. When the mute signal is no longer pulled to ground, the radio returns to the mode it was in previously.

Hooking up a professional or factory scan tool that can read radio codes would be one of the first test's to do .
DTC B1025, B1035, B1045, or B1055 (w/out RPO WBM)
DTC B1025, B1035, B1045, or B1055 (w/ RPO WBM)
DTC B1030, B1040, B1050, or B1060 (w/ out RPO WBM)
DTC B1030, B1040, B1050, or B1060 (w/ RPO WBM)
DTC B1259
The following conditions are present for 250 ms or longer:
B1025 - B1055 01
One of the speaker positive output circuits is shorted to battery.
B1025 - B1055 02
The speaker positive output circuit is shorted to ground.
B1025 - B1055 04
The speaker positive output circuit is open.

Your best bet , take it to a qualified repair shop .

* 12 Volt system; 4 + (bulb 2 + bulb 3) = 12 volts
bulbs 2 and 3 will total 8 volts.
* Unusual set up for auto lights to be in series. the usual is the full 12 volts for each bulb.
* A series CKT would mean if one bulb failed, they all fail. (like old Christmas lights, you have to check them all to find the bad one.)
* Wattage of bulbs will have an effect on the brightness of each bulb. Higher wattage will affect the voltage drop across that bulb.
* if your trying to "drop" a higher voltage to a low voltage device. Then you need to take in account of the device "loading" to get the right voltage and power (amps) to also drive the device.
* You will also find that using a bulb as a voltage drop will change with the current draw. Higher draw will increase the voltage drop across the bulb (it will get brighter and lower the voltage on your device.)
* In series resistance was used (in old days) as a voltage drop for arc lights or even arc welding. If the line shorted then the resistance coil (nichrome wire) would take up the load, preventing the breaker from tripping out on a short.
Aloha, ukeboy57

1979 1600cc what? Early chevy cars (1963 for example) had a resistor wire between the coil and the ignition switch that took the place of a resistor coil. The theory here is that when cranking to start, you want the full 12 volts that by ohms law means you have lower amps, but higher voltage to jump the cold spark plug gap and make for easier starting. Once running, you have a resistor that reduces voltage but increases amps, to get a fuller burn and better efficiency. THEREFORE, you can test for voltage being supplied to the coil at "key on" to see which coil you need. If you have full 12 volts, buy resistor coil. If more like 6.8 or so, use non resistor coil.

INSPECT BATTERY ELECTROLYTE LEVEL
INSPECT BATTERY CABLES/TERMINALS
LOAD TEST BATTERY,_______VOLTS @ 150 AMPS (Min SPEC 10.2 VOLTS @ 150 AMPS.)
INSPECT BELT(s),
TEST ALTERNATOR OUTPUT (ON THE VEHICLE,'NOT' BENCH TESTED),
1 _____AMPS @ 12 VOLTS @ 2500 RPM w/PLOW not ENGAGED
2 _____AMPS @ 12 VOLTS @ 2500 RPM w/PLOW ENGAGED ALT OUTPUT SPEC ????

If you need further help, reach me via phone at https://www.6ya.com/expert/ralph_2f0886b7d6af32ce

In some brand of alternators the regulator and brushes are an integral part and it is a matter of removing 2 screws and lifting out the part. On others the regulator is entirely separate unit located some where else on the vehicle. The voltage of 15.3 is on the upper end of the charge rate but it would be acceptable . Nominal voltage for a regulator is 14.5 to 15.5 volts. Understand that a battery is actually 13.2 volts not 12 volts and you need 14 volts for the battery to charge effectively. The higher the voltage the higher the amperage to run everything so if your alternator regulator is set at 14.5 volts then the max amps of the alternator would not be reached and if you have a current draw to run everything that is more than the output of the alternator then the battery will eventually go flat. Find yourself a reputable auto electrician and discuss the problem with him not a local mechanic

I have a thought you may try testing your battery voltage with the car turned off .It should read 12 volts or higher if not charge battery than check again if battery is lower than 12 volts after charging replace battery.If 12 volts or higher start engine check voltage on battery again it should be 13 volts to 14.5 volts not higher if voltage readings are lower than 13 volts or higher than 14.5 volts replace alternator.

amps or volts
11 to 12 amps indicates nothing but if it is volts then it indicates that the alternator is not charging the battery
Battery voltage is 12 -13.5 volts and when charging is 14.5-14.8 volts dc
the rest of your statement indicates that the alternator or voltage regulator is faulty and you should have the charging system checked by an accredited auto electrician

Most likely cause is heat. The clearances behind the radio are often not sufficient to provde the proper air coolling that the higher power radios need. The heat sinks on the rear of the radio require a certain amount of air flow to dissipate the heat generated under high load conditions. If you add summer heat to the equation, you can have the radio going into thermal cutout to protect it circuity.

The radio volume goes up when you give it Gas this intern hides engine noise. This is very common with higher end cars
There is a adjustment on the radio that will set the level you wish the volume to go up when accelerating, its a nice factory option to have there is nothing wrong with your BMW.

Check to see what th voltage is at the bulbs. It should not be over 12 1/5 volts. If higher, install a resistor in the circuit to lower the voltage. Radio Shack will have resistor. If you are in Europe, find a hobby store, or coil up about 20 ft of wire to drop voltage