This is how I fixed my P3100 that was stuck on mute.

I turned off the unit by holding the source button.

Tapped the screen and "AV Menu" appeared in the top right of the screen.

Tap it then go to the first choice which should be "Initial Menu"

you will see "TEL" then off to the right of it it should say "Mute"

Change "Mute" to "OFF" and your issue should be fixed.

 

I think mine had to do with a conflict with the steering wheel controls adapter. Its the only logical explanation I could come up with.

 

I assume this would work for any of the AVH models.

 

 

Post up if this helped you at all.