My friend had the same problem with his Avic-N2. The fix is pretty simple. It seems that he did not ground the MUTE wire properly (I beleive it is the Black/Yellow wire coming from the harness). The wire is usually connected to your cellphone kit if there is one installed. What happened is that when he would hit a bump of some kind, the wire was shorted against something causing the unit to think that the was a MUTE singnal sent from the cellphone kit. All i did was, i removed the unit and properly isolated this wire. It's been 3 weeks since and NO PROBLEMS. Try this, it should work. !!
Hope this helps !!