That's right about mac iTunes. It doesn't recognise all mp3 formats. Such as Ogg Vorbis.
For mac the best alternative is Audion which is now free & recognises all mp3 formats (though not sure about rm.) Audion:
http://www.panic.com/audion/
For windows I'd suggest the best backup player is QCD:
http://www.quinnware.com/ (it's free & has built-in codecs & plays ALL audio formats!!!)