Hmmm, have you tried going to
'System-Administration-Login Screen'
.Then press the unlock button, and type in your password, then uncheck the "Play Login Sound" check box. That should turn off the startup sound.
I don't know whether that will work, because I have ubuntu 10.04, but it should be roughly the same...
Image here:
HERE
Mind you, thats only a temporary fix...
If you have Audacity and you know where the startup sound file is, then you could open it up in audacity and turn down the sound by going to 'Effect-Amplify' and push the slider to the left about 10-20 decibels...
Ill see if i can find where that sound file is...
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