I burned another DVD+RW with Nero and it played successfully in the Philips DVD 642. Roxio 6 seems a poor choice for burning MP3s with broad compatiblity. It doesn't allow enough options to fix compatibility problems.
If you don't have Nero, you can get a trial version at Nero 6.6 Trial Version. For best compatibility use the following settings:
File Name Length: Max of 11 = 8 + 3 chars.
Character Set: ISO 9660
Relax ISO Restrictions: All unchecked
Relax Joliet Restrictions: Checked (shouldn't cause a problem)
If your player can read long filenames, they are still present in Joliet format and will be read. If your player can't read long filenames, it will read the ISO 9660 short filename format and play them.