In Internet and File Exchange, Real Player tends to take over a number of formats it can't even play, including mp3. The first step is to make sure that iTunes (version 1 of which you can use BTW with a patch) or whatever your mp3 player is, is selected for that file type.

Even then, Limewire will often ignore these directions. If you have iTunes, there is a little freeware prog. at versiontracker called iTunes Associator. If not, then your only choice is to change the file type and creator with ResEdit, Norton Fast Find, or similar program to the type and creator of your mp3 player.