You might have Digital Copyright Enabled music. If you use Windows Media Player on a PC to copy your music do the hard drive, it will add a licecne key into those files so they can only be played on that computer. Hell, if you change your hardware, like replace a bad modem or network card, the music wont play anymore.
If you DO have the Digital Copyright thing enabled, you will need to go into WMP's settings and disable it, then eiter re-copy your music, or remove the licences. I think there is a way to add licences to your mobile devices, but i dont know anything about that. I hate the whole fascist Windows regime. *farts in Microsofts general direction* :P
A great CD to MP3 compressor is CD_Ex. Its simple to use, works fast, and the best part... its FREE. It even goes online to the IMDB server and renames/labels the CDs you rip to MP3, and puts them in proper folders. Its really a simple and painless program.
You can get it here
http://cdexos.sourceforge.net/
or
http://sourceforge.net/projects/cdexos
