Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_1 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8B117 Safari/6531.22.7)
Not sure if you can turn it off alltogether, however, you can set your message sound to "None" in the sound settings menu. And you can turn "Repeat" off, under the message settings.