I don't have my mac in front of me but I think you want to do this:

Right click on an mp3 somewhere and select "Get Info." There should be an option in the Get Info window to set the default program associated with that file type.

I'll check to make sure that that's correct when I get home.

- trs

**EDIT** : In the Get Info window, There's an "Open With" section where you can set the default app for that file type.