Hi, Just a general note here:
If you want to change the settings for any default application, (such as the default mp3 player), open "file exchange" in your control panels. You can change them there. Also, you can do the same in your internet browsers by changing the "helper applicatons" in the preferences.
To answer your question/problem:
Just find the .mp3 extension in the File exchange control panel, click on "change." then locate iTunes and select it.

Pam