Probably the media path has been changed. Launch Supermemo and open the option-panel (Ctrl+P). In the 'Secondary storage' section on the 'Data access' panel type the correct path to your DVD drive where the DVD-ROM is placed.
If it doesn't help, you will have to reinstall Windows Media Player 6.4. The installation program is placed on the DVD-ROM in the WMP6.4 directory. It will reset the default drivers and audio codecs settings.