Most of the time it's just an annoyance, like in the clip below. But other times the sound volume jumps A LOT. To the point where I need to lower the speaker volume.
I just played a game with simulating world in the background disabled, but it still occurred.
I'm using a Audigy 2 platinum with XP pro. All my drivers are up to date.
Here's a quick 15 sec clip from my last game. This is mild compared to some of the increases, but you can hear the sound jump up and down a couple times.
http://www.edfick.com/ootp/ootpsoundbug.mp3