If you are booted up in classic, you should be able to set the doubleclick app. for mp3's. Go to the Control Panel "File Exchange," scroll down the list to select ".mp3," then hit the "change" button to choose the app you want. You have to set OS9 as your startup disk for this to work.

But since you are probably booted up in OSX, here's what Apple says in http://docs.info.apple.com/article.html?artnum=106171

Steps for Mac OS X 10.1 and later

1. Click the document's icon in the Finder to select it.
2. Choose Show Info from the File menu.
3. Choose "Open with application" from the upper pop-up menu in the Info window.
4. Choose the desired application from the lower pop-up menu in the Info window. You may choose Other to select an application not on the list.
5. If you wish to open all documents of this type with the selected application, click Change All.

There's also an app that you can use, like FileType http://www.frederikseiffert.de/filetype/

Well, hope these guesses help. Let me know what works, and keep smiling that you've the time to play with this stuff, and the interest to bother.