Ringtones Stored as AAC Files on iPhone
There isn't a whole lot revealed in the iPhone's file system contents, but one tidbit is that Ringtones appear to be stored as .m4a (unprotected AAC) files, suggesting that adding custom ringtones would be as simple as copying AAC files into the appropriate directory.
/Library/Ringtones
/Library/Ringtones/Alarm.m4a
/Library/Ringtones/Ascending.m4a
/Library/Ringtones/Bark.m4a
/Library/Ringtones/Bell Tower.m4a
/Library/Ringtones/Blues.m4a
/Library/Ringtones/Boing.m4a
/Library/Ringtones/Crickets.m4a
/Library/Ringtones/Digital.m4a
/Library/Ringtones/Doorbell.m4a
/Library/Ringtones/Duck.m4a
/Library/Ringtones/Harp.m4a
/Library/Ringtones/Motorcycle.m4a
/Library/Ringtones/Old Car Horn.m4a
/Library/Ringtones/Old Phone.m4a
/Library/Ringtones/Piano Riff.m4a
/Library/Ringtones/Pinball.m4a
/Library/Ringtones/Robot.m4a
/Library/Ringtones/Sci-Fi.m4a
/Library/Ringtones/Sonar.m4a
/Library/Ringtones/Strum.m4a
/Library/Ringtones/Timba.m4a
/Library/Ringtones/Time Passing.m4a
/Library/Ringtones/Trill.m4a
/Library/Ringtones/Xylophone.m4a
The original site has asked not to be linked, but more information can be obtained via IRC (#iphone on irc.osx86.hu)

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