GEt a copy of MP3Rage from chaoticsoftware.com (it's a free demo). Drop the bad file on it, and use the popup menu on the right (right next to where it says Type and Creator) to select the type of player you want the file to open with.

Works for me every time. You can also use MP3 Rage to fix Stuffit files (not just MP3s) that show up as Limewire documents: just change the Type to SITD and the Creator to SIT! (all caps, and you do need the exclamation point)