Here's some info from the OSX help menu (wasn't easy to find!)
"Changing the application that opens a file"
"You can select a different application to open a file. You may want to do this if you prefer using a different application or you don't have the application used to create the file.

Select the file in the Finder and choose Get Info from the File menu. Click "Open with" in the Info window. Choose an application in the pop-up menu or choose Other to select an application not in the pop-up menu.

To change the application that opens all documents of the same type and created by the same application as the current document, click Change All."

just make sure you choose the OSX iTunes--you might have the OS9 one on your machine too.