There’s nothing worse than firing up the Mac at a library and the startup sound is cranked up. Oops! Sorry!
For some reason, there is no option to disable it in the operating system.
Try downloading and installing this small app:
It’ll add an option in your system prefs to change the level or even disable completely the the startup sound.
It looks like development on it has pretty much ceased, but it’s working fine for me in Snow Leopard.