clipped from: www.iphoneatlas.com   
Though this capability might well be removed in a future version of iTunes, it’s currently possible to turn any MP3 or AAC file into an iTunes ringtone file that can be synchronized with the iPhone.
For Windows users,
Using Windows Explorer, duplicate and rename any AAC with the extension “M4R”
  • Double-click on the file. It will be added to your iTunes library automatically.
    • Click on your iPhone in iTunes, and go to the ringtone section. You should now see your new ringtone.
    sync
    For Mac users,
    Duplicate and rename any AAC file, then select it and choose “Get Info” from the File menu. In the “Name and Extension” section, change the extension to .M4R
  • Double-click on it. This file will be added to your iTunes library automatically.
  • Click on your iPhone in iTunes, and go to the ringtone section. You should now see your new ringtone.
    • sync
    if you’re starting with an MP3 or WAV file, you can first select it in iTunes, then choose “Convert selection to AAC”