anfmanusa,
Most MP3s are not CD quality because they are not lossless. If you want true CD quality sound or higher you need to go with a different audio file type such as FLAC or SHN. So with a MP3 player + FM Modulator you decrease your sound quality a great deal.

http://flac.sourceforge.net/
http://research.umbc.edu/~hamilton/shnfaq.html