No sound when playing DVD's; CD's okay

Does anyone know how to trouble-shoot for audio when playing a DVD? I have had no problems with sound. I have been listening to itunes and Pandora all day.

Here's the scoop:

1. I burned a copy of a film screener (with permission). The video was fine, but it had no sound. (I have made a copy of this before and it was fine.)

2. I tried another DVD of a music video. Again, the video was ok but not the audio.

3. I tried a purchased movie DVD. The audio still didn't work (but video did).

4. I tried a CD and the audio and video worked.

PS Not sure if this is important but thought I should mention: For some reason, the video of the copied DVD kept popping up when I inserted the music video DVD. Once I restarted the computer, it played the correct DVD loaded in the player.

Thanks for any suggestions.