I'm not really sure how to explain it. It's always been my default player. I think you can go to File Exchange (Under the Apple Menu) and select mp3 extension and then edit and choose QuickTime as default application. I'm sure there are more detailed instructions here in the forums. Try doing a search of these forums for "Setting default player" or "open in quicktime" something like that and you might find some more useful information.

Good luck and sorry I couldn't help more.