A word of caution, first: Apple doesn’t sanction these hacks, so if something goes wrong, chances are the Genius at your local Apple Store will be unsympathetic to your woes. Again, this hack worked just fine for me, but if this sort of thing makes you nervous, best leave your $499 mobile phone with its standard ringtones.

see more http://www.macworld.com/2007/07/secrets/iphone_ringtones/index.php