Factory cd players may be adhering to copyright music. WAV files are used in original CDs and easily played in vehicle factory cd players. However, if you tried to play mp3 CDs, they won't play. WAV files are larger while mp3 files are compressed. Aftermarket cd players will specify wav and/or mp3 file compatibility. Many aftermarket radios may have an audio mp3 input port to plug in portable mp3 players for non stop music. Try a cd with mp3 music and if it doesn't play, you'll have to convert mp3 files to wav files then burn a cd to play in your cd player.