BTW, what's the difference between a regular mp3 file and MPEG Layer 3
Do you mean MPEG-1 Audio Layer 3? If so there is no difference. MP3 is the common name for MPEG-1 Audio Layer 3.
I'd like to have just ONE Player that can play all of my files without me having to do conversions. Is there a program that can accomplish this?
It all comes down to licenses. RealPlayer, for example, won't release their code to developers so it's very hard to incorporate it into a third party application. Some developers get round this by "emulating" a RealPlayer player or plug-in for another application.
Winamp is probably your best option, then if you have a file format that it won't play, search through the
plug-in section to see if someone as written an add-on.
I can't help you on the Mac issue though as I've never used one.