Don't sweat the headphone problem - it's not something many people are aware of, and even having chased it down before I found myself scratching my head once a couple months ago after holding "mute" down while my daughter spun the volume dial with reckless abandon.

The volume changing is definitely odd. I've heard of some plasma TV's interfering with IR signals, but not of them or other components generating signals. Do you have an RF-to-IR device in the system anywhere?