no video on certain dvds

Hi there
not sure if this is the spot or not but I'll lay out my troubles here first. I have a Dimension 3000 machine with 512mb RAM, XP pro, P4 2.8 and all the updates. When using the DVD player, the occasional movie will not produce audio. speakers are working because the Windows startup sound works fine. Everything except the audio is alright with this dvd. next one works fine. Question is where do i start to diagnose the problem as everything seems to be fine? thanks in advance for your dieas/suggestions
Brent