Ugh, are you serious?
That's one of my biggest (even though it's really minor) complaints with my Triumph. When I'm listening to music and a notification sound plays, it drops the audio waaaay down. Sometimes pausing the music for a bit and starting it back up fixes it, but other times it just stays there.
This happened on stock ROM, and various versions of CM7, so it must be a very basic hardware driver issue.