Audio won't play for DVDs on VT7

I built a PC for a friend that includes an NEC 2500 DVD burner and a
Liteon DVD/CD-RW. I used an ABIT VT7 MB with a 3.0GHz CPU w/512MB
mMushkin DDR PC3200.
Both optical drives play audio just fine from an Audio CD (MP3 or
wav), or a DivX file.
Anytime I insert a DVD (retail or cloned) though, the audio won't
play. The audio won't even play through DVD Shrink's preview.
I've tried using audio cables or not.
I'm using an 80-pin IDE cable.

Any suggestions?

-Jaze