Change Your System Sounds!
For some reason, only .caf and .aif sound files are valid formats for system sounds. You can use iTunes to convert files to .aif format. iFuntastic's sound editor also produces .aif files. GarageBand works very well to create original sounds: drag songs into it, crop them, and fade in and/or out as desired; you can mix them, you have all those nifty instruments and samples, and you can record voice - what more could one want? Drag your sounds into the list or choose 'Replace With...' from a sound's popup menu. iFuntastic makes an automatic backup to _org_[file name]. You can restore the sound from this backup with the 'Restore' selection in the popup menu. System sounds are different from ringtones in that their names are fixed. This means you'll be replacing the contents of the file only. iFuntastic will make a note in its manifest and display the name of the new file in a note below the changed sound. It's probably not a good idea to delete system sounds - rather, you may want to replace them with the 'SystemSoundOfSilence' that can be found in the 'System Sounds" folder. Click on the speaker icon to listen to a sound. With most new sounds, you will have to restart (sometimes even reboot) the phone, before it will play them.