The sound is not working properly..

Take care. First put the volume down complete. Then unplug the head phone and than try to raise the volume. Could be the headphone connector does not switch through correct. Could be the connections between the pre and power amp are not in place or not proper pushed in?
Always take care when working with amplifiers. When switching them on with volume to max, you easy can kill your speakers. Also when disconnecting the headphones on full power kan damage the loudspeakers.

I know this will sound stupid but try it, unplug just power cord, leave it unplugged for 2 hours then plug it back in. This will reset it to factory setting and if this issue was caused by a power issue it will fix it.

try dialing a uncharged no like free customer care then raise the volume by pushing the up arrow repeatedly

what type of phone do you have

check your manual for a quiet mode, or reset factory defaults. it might be stuck in "attenuator" mode, where sound is muffled, in case you're answering a phone.
Good luck!

The most likely cause is a broken earth wire and or bad connection

This will sound strange, but some DVD players and cable boxes have separate volume controls. I would check the settings for both of those to make sure that the volume has not been turned down and also check the connections from the dvd player and cable box to the tv. Make sure all connections are solid.

when anyone ring us, we can't hear them they are faint . they can hear us fine. is there a function on the phone or machine were i can turn the volume up to hear them ???

I suggest if your handy , to open the receiver and look for a circuit breaker or a fuse that protects unit of being damage , replace it and turn it on , hopefully it will work .

If the sub has a volume control, turn it down then turn up the volume on the amp to the very high volume you mentioned. If it still has very low muffled sound, then we know the problem is about the amp.