Comment 3 for bug 968258

The easy (but probably non-permanent) solution is to just remove the sound file, as outlined in http://askubuntu.com/questions/66786/how-to-disable-login-sound/66788#66788

Punch this into your shell:
sudo mv /usr/share/sounds/ubuntu/stereo/system-ready{,-disabled}.ogg

A nice checkbox somewhere would make a lot more sense though.