Custome sms or any other system sound

Firstly the sound file has to be in a .caf format.
Convert any sound you want to .aiff, then rename the extension to .caf and replace the file you want.

rename the file to one of the following:

place the file into:
System/Library/Audio/UISounds (you can make a back up of the original sms-received file if you like)

this will replace the file on your phone with the sound you want. the caveat is that on your phone it will still show up with the name of the original file, i.e. if you replace sms-received4.caf it will still read "Horn" on your phone in settings. but when you select it will play the tone you replaced it with. of course to reverse this just replace the original file in the folder

PS. I made a little [ame=""]youtube guide her[/ame]