Here’s the How To:

1. Duplicate and rename any AAC file (or convert other file types to AAC via iTunes).
2. In the Finder, select it and choose “Get Info” from the File menu
3. Change the file’s extension to .M4R
4. If it is already in your iTunes library, delete the original from your iTunes Library (re-import the original back to your library when you’re done)
5. Double-click your .M4R file to automatically add it to your iTunes library
6. Click on your iPhone in your iTunes “DEVICES” list and select the “Ringtones” tab – you should now see your new ringtone
7. Sync to add it to your your iPhone

This trick keeps working even after iTunes 7.4.1 Update.

Posted in Macintosh Tips & Help, iPhone & iPhone 3G