OK this happens for me with both mga1 and mga2 64 bit.
I was hoping that the update to mga2 would see the back of this problem. (fresh install by the way)
if i run videos, avi's mkv's , mpgs through vlc i get a kind of sibulence over the top of the sound.
however other video players such as mplayer do not have this issue.
vlc also has this problem with plain old vanila mp3's.
using it with sound blaster audigy 2.
this has to be something to do with the audigy as another mga1 PC with different sound card does not have this problem.
regards peter