I always forget how to disable the “login ready” sound of the drums.

System >> Administration >> Login Window
Click on the Accessibility Tab
Uncheck the Login screen ready sound box

OR there’s the login and logoff sounds:

System >> Preferences >> Sound
Click on the Sounds Tab
Disable the Login and Logout sounds

***EDIT***
Apparently things are a little bit different in 9.10.

System > Preferences > Start Up Applications
Deselect the Gnome Login Sound

Now you will no longer hear the login noises.

Open up the terminal
sudo -u gdm gconftool-2 –-set /desktop/gnome/sound/event_sounds –-type bool false

Now you will no longer hear the startup noises.