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How to convert AAC to M4r manually using iTunes 7.4 (Windows, Mac)?
That ringtones and song files are only distinguished by file extension. The file extension for Ringtones is "M4R".
1. To add a ringtone to your iPhone, using Windows Explorer or Finder just duplicate and rename any AAC file with a "M4R" extension, then double-click on it. This file will be added to your iTunes library automatically. (Some are reporting a size limit for the music file, so if you have trouble, using a smaller file may help.)
2. Now, click on your iPhone in iTunes, and go to the ringtone section. You should now see your new ringtone. If you want to be sure about the sync, feel free to click "selected ringtones" and check off the file explicitly, though this shouldn't be required.
3. Now SYNC your phone. You're Done!