100% GUARANTEEHere is the method to add you own custom ringtones to your iPhone.
1. Install iTunes 7.4, restart the system.
2. Open iTunes
3. Convert a song.mp3 to AAC (from your playlist)
4. This song will be in the /iTunes Music/
(if that the default iTunes music folder)
5. Now let's convert that song to a Ringtone. We need to change the .m4a to .m4r (very simple).
5A. If you don't see .m4a, go to Control Panel/Fonder Options/View/
Uncheck [ ] hide extensions....etc.
6. After you convert it to .m4r, open it, it will be in your iPhone Ringtone Folder.
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.
Credit to: Macrumors
Forgot about that.