If it always did this, time to ask it's maker.
When every I try playing DVDs
on my Windows Vista Computer there is no sound no matter what program I
use. I've tried using Widows Media Center, Real Player, Windows Media
Player, VLC and a number of others. I can't get sound out of any of them. Its only with DVDS other then that the sound works fine.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic