For this work your phone must be jailbroken.
- Open itunes and go into preferences.
- Under the general tab click the import settings button and change import using to aiff encoder.
- Right click your text message sound (add the sound to your library if you haven’t already) and go to create aiff version.
- Rename the file to sms-received6.caf (if you need it).
- Using whatever method you want put the file into your iphone under /System/Library/Audio/UISounds/ (i use iphone explorer). Overwrite the old file.
- On your phone go to your text message sound settings and click the last text message sound. You should recognize it.