This comes up periodically on these forums, so a search might turn up other suggestions.
In my memory, that should set the volume based on the volume when the computer is shut down, so turning the volume down before turning it off would help.
Alternately, if you have speakers/headphones plugged in, the sound will only be played through those. Thus, plugging in headphones, or turning off your external speakers, will effectively silence the startup chime.
I'm not aware of any way to turn it off in firmware, but maybe you can...