The simplist way of doing this job is with a jail broken phone. First SSH into your phone and go to…
/private/var/mobile/Applications/
you are going to see a lot of folders with weird names. Unfortunately you are going to look through them all one by one. The best way to quicken this process is in your ftp client, to organize the folders by last modified.
So keep on looking in all the folders until you find one that looks like this
look for a folder that contains the sub folder /Fring.app/
now go into the folder and look for the file Message.wavand rename it to something like Messege.wav-stop.
Download this Messege.wav (right click) which is essential an empty file. Upload this file in the same folder where you found the original Messege.wav, and you done.
thats it, dont need to reboot or anything