Windows, apparently, has a built-in audio ducking feature which is very annoying (to me at least). When two or more audio sources are playing and the maximum audio volume exceeds 100% the OS applies a limiter instead of clipping the audio. It has nothing to do with the Communications settings (Right Click Speaker Icon -> Sounds -> Communications tab) in case you think it is. Windows XP won't do that. I didn't test it on Vista.
EDIT: I found that the component I'm talking about is called CAudioLimiter