There is no such thimg as XviD audio. Movies are generally encoded with MP3 sound (than can be decoded by Windows, no special codec requiered) and AC3 sound wich can be played using an AC3 decoder or filter. I think that the second one is your case. So, just install an AC3 codec or filter:
http://forum.bsplayer.org/viewtopic.php?t=3042