Problems playing DVDs

[Master.America]

A few choice DVDs run just fine, but most have trouble. Either the sound is disproportioned (I can hear background sounds and music, but voices are too soft to be heard) or the video/sound skips, as though the DVD is in double-time. I'm using my Toshiba Tecra A4 laptop. It uses a Mobility Radeon x600 for video and a Soundmax Digital sound card for audio. I've tried various drivers for both, but to no avail. Both problems persist.

Help, anyone?