perhaps you should try a different DVD playback sofware - the one you're currently using may not like your system setup or is conflicting with something. This is very possible if same playback software works fine on a different system (your laptop).
I use a sony drx-710ul dvd burner. When i try to play commercial dvds on my desktop(WinXP) I can see the images but there is no sound. CDs and home videos work well. Also works great in my HP laptop which also uses WindowsXP. I have tried new and old DVDs and its all the same. I tried connecting through USB 2.0, and 1.1--nothing. Changed the USB cable--nothing. Updated the firmware--nothing. Contacted sony tech support and they told me to do all of what I had tried. Nothing new. I use a 5 port 2.0 USB card. Sony tells me that the device sometimes do not work well using USB hubs. I tried a firewire connection and all this did was to crash my computer everytime I tried to play the DVD drive. I am dying to fully experience my new flat panel monitor and surround speakers and it has been frustrating.
Thanks in advance for your help.