The work around for this issue is to replace one of the 6 new tones with your own. First, you'll need to convert your sound to a .caf file. Then using your favorite browser program (WinSCP, iBrickr, Cyberduck, etc), navigate to this directory on your phone:
/System/Library/Audio/UISounds
The 6 Apple-created SMS sounds are as follows:
sms-received1.caf = Tri-tone
sms-received2.caf = Chime
sms-received3.caf = Glass
sms-received4.caf = Horn
sms-received5.caf = Bell
sms-received6.caf = Electronic
So you'll want to pick one of these to replace over. For example, if you decide to replace Electronic, you'll want to rename your sound file to sms-received6.caf and just upload the file to this directory. Then go to Settings-->Sounds-->New Text Message and select your sound file.
Please note that the SMS name will not change. So if you replaced Electronic with your own custom tone, it will still say "Electronic" in the menu, but will play your custom tone.
P.S.: For the .caf file use a program that converts to .aiff then change extension to .caf.
kthx
