[rstoyguy]

Open your favorite registry editor and navigate to the following key:

HKEY_CURRENT_USER\ControlPanel\Notifications

Next, scroll down through the RGUID's until you recognize a value of:

"Default"="MMS_something you recognize.... "

Then delete the whole RGUID Key to remove the sound. And if your wondering an RGUID will look like:

{A877D659-239C-47a7-9304-0D347F580408}