There's many types of mp3's. Some are encoded with copyright formats. To play them you'd need those particular codecs. There's also mp4 (m4a) which is a new type that's showing up but iTunes can play these. I'd suggest you get a hold of another mac mp3 player such as Audion. I've found Audion can play these other types of mp3's. You haven't told us whether U use OS 9/X or what application you tried to play them in.