Comment 24 for bug 437429

I was able to remove the login sound (not the drums but the successful login sound) only by doing the following:

1) Go to System > Preferences > Startup Applications
2) Uncheck "GNOME Login Sound"
3) Right click the speaker icon (volume applet) and select "Sound Preferences"
4) In the "Sound Effects" change the "Sound Theme" to "No sounds"
5) Reboot

Result: No login sound

Note:
1) If you do not change the "Sound theme" to "No sounds", the "GNOME Login Sound" entry in "Startup Applications" will reactivate on its own.
2) I am guessing you would have to do this each user on the machine.