How To Add a Custom Ringtone on the iPhone
The 23 step tutorial requires an involved process which is not without risk:
Following these instructions may brick (kill) your iPhone making it completely unusable. If your iPhone becomes unresponsive to restoration via iTunes, then you may have to seek repair or replacement through Apple Inc., or AT&T. However, everyone we know of has been able to restore their iPhone fully by using iTunes. Pay careful attention to the instructions here. We are NOT responsible for your results. You are assuming all risks. These instructions were completed successfully by many people before being posted here, but we cannot account for everyone's individual computer system configurations or ability to follow directions.
As it turns out, a few MacRumors forum members have successfully installed ringtones with this procedure.

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