I have no clue, so mine is a guess:
-hardware accelleration
-some games have a soundsetting which enables 16 or 32 bit mixing? might be that the game is set to 32 or something and ventrilo overrides the game to clear some soundchannels?!
I really dont know how they work, just brainfarts here m8ey.. hope they make sence.
Woof ~ Woof