A few students have had this problem in the past, and each time it was because of the type of DVD player being used.
Older DVD players can have problems playing the sound on some of the DVDs.
Before you open a support ticket (by clicking any support link on this page), I want to make sure it really is the DVD's causing the problem and NOT your DVD player.
Can you try playing the DVD's on another DVD player? There may even be a DVD player on your computer... or you can take it to a friends house to test it on their DVD player or computer?
If it ends up being that the DVD's ARE bad, please click any support link on this page to open a support ticket and I'll send you replacement DVD's.
Make sure to include your order ID or email address you put on the order.