Help with Pioneer DVD-RW DVR 115D sound problem?

I have a pioneer DVR 115D and it works fine when I playing audio cd's. Problem is though when I play dvd's - I get a great picture but no sound! I've checked all the boxes and none of them are on mute, there is no conflict. I've tried windows media player, real player, quick time and the software included in the Disney dvd's but nothing works. Any idea's as to what is the problem? Weird thing is though the extra's play fine and I can use all the "extra" features contained in the dvd rom (like games and stuff) but can't get any sound on the main feature film.

I've gone through all the suggestions that help centre has offered and googled all night so far but nothing is helping. It's probably something simple but I'm missing it!

I've got a

Creative sound blaster PCI 128

Operating on windows XP professional with 1MB RAM

The computer is from Dell and it's only 6 months old but to be honest this is the first time I've tried a DVD on it and I'm puzzled. I don't need to update any firmware etc and am using the XP driver.

It works with DIVX dvd's and any other movie I have installed on my hard drive but according to the player there are "no sound files" on the shop bought dvd's..........

What's going on?!