Having some recent, odd problems with my Xbox 360. (Not in warranty, about to break the cases open on the box and brick to see if I can figure or change/replace anything to fix it... )
When using Composite or HDMI in my TV or Monitor, I will sometimes loose audio for a fraction of a second, than get a huge loud burst of audio all at once, louder than what my volume settings currently are.
This happens quite literally in less than a second, and happens randomly. Sometimes within a few minutes, other times it wont happen for days at a time. Happens in all games, very different display, tried multiple composite and HDMI cables, even went as far as switching power-strips and testing on different circuits and friends houses. Same thing, every time.
TV audio, or headphones, and sometimes both. Switched power supplies(bricks), nothing changed. Only thing I cant accurately test, is using another Xbox 360. I have a different generation and model then what my friends use.
Only time the burst of audio doesn't happen, is when I'm on the dashboard listening to music, or running through the dashboard apps. Other downloadable games though, Including Call of Duty Elite, Halo Waypoint, etc, the sound occurs in them as well.