This is one reason why using ac3 audio on your DVD is a bad idea - sooner or later it will be traded out on cd. It's much harder to distinguish ac3 converted to wav than it is mp3 converted to wav.

Using a "what you hear" type soundcard means that every other sound on your computer will be recorded along with the audio from the dvd. Always wanted to hear "You've Got MAIL!!!!!!!!!!!" in the middle of Corgan singing Rhinoceros or whatever.