My incoming call volume is set high but still I don't hear any ring for an incoming call. Further, when I switch on my speaker, the volume is very very low, so much so that I cann't hear anything. Earlier, it all was working fine. I don't know what has gone wrong suddenly. May be volume is locked or some moisture has seeped inside... Pl. help.

The ringer circuit could be defective.

Have you tried to call the phone and adjust the volume on the set while the call is coming in? I think the only way you can adjust the ringer volume is if the phone is ringing.

See if this helps

Adjusting the Ringer, Earpiece and Speaker Volume

Adjusting the Ringer Volume

You can choose from four ringer volume settings (off, low, medium, and high)

separately on each handset or the base.

With the phone in standby, press or on the handset or VOLUME or

VOLUME on the base. The handset or base sounds the ringer at that level.

Use on the handset or VOLUME on the base to make the ringer volume

louder. Use on the handset or VOLUME on the base to make it softer or

turn it completely off.

When you hear the ringer volume level you want to use, stop pressing keys.

The handset or base will use the new ringer volume starting with the next incoming call.

Note: If you set the ringer volume to Off, the handset or base won't ring. Other handsets or base will ring according
to their individual settings.

Check all tones settings and adjust the volume, check incoming alert ringing, or your speaker need to replace.

first of all u can increase volume during call by pressing right button (middle one). but i think ur phone is set to max and u dont hear much - in this case probably some "metal dust" sticks on your speakers membrane - open case clean speaker gently or replace with new one...

Your HP all-in-one provides three levels for adjusting the ring and speaker volume. The ring volume is the volume of the phone rings. The speaker volume is the level of everything else, such as the dial tones, fax tones, and button press beeps. The default setting is Soft.
1 Press Setup.
2 Press 4, and then press 8.
This displays the Basic Fax Setup menu and then selects Ring and Beep Volume.
3 Press to select one of the options: Soft, Loud, or Off.
Note If you select Off, you will not hear a dial tone, fax tones, or incoming ring
tones.
4 Press OK.

I found this at hp website.

Adjust the volume
Your HP all-in-one provides three levels for adjusting the ring and speaker volume.
The ring volume is the volume of the phone rings. The speaker volume is the level of
everything else, such as the dial tones, fax tones, and button press beeps. The default
setting is Soft.
1 Press Setup.
2 Press 4, and then press 6.
This selects Basic Fax Setup and then selects Ring and Beep Volume.
3 Press to select one of the options: Soft, Loud, or Off.
Note If you select Off, you will not hear a dial tone, fax tones, or incoming ring
tones.
4 Press OK.

You can set the length of your incoming ring to in the Call forwarding portion of the phone. Usually, it is set to 25 seconds. It can go as high as 30 seconds. To set the ringer to be longer or shorter, it goes by 5,10,15,... set of 5 increments. up to 30.
I would also recommend to use a factory set ringtone, set the profile to LOUD or OUTDOORS. With vibrate option on. Some downloaded ringtones don't play as loud.

your speakerphone may have been damaged

Hi at the back of the phone you will see a selectable Hi/Lo/Off switch. Slide the (Ringer) switch to High position or Low position to turn the ringer on. Hi for high volume Lo for low volume.

To prevent hearing any ringing of incoming calls slide the (ringer) switch to the off position but when you ready to accept incoming calls again remember to slide the switch back to either Hi or Low setting. Hope this made sense and helped.