"the sound" that I think he wants to suppress is the startup chime, which can't be turned off because it serves a very specific hardware debugging purpose related to the boot process. There are hacks that can turn down the volume of the chime but it cannot be completely turned off.

Strange. When I mute my computer before rebooting or shutting down, it does not make the sound.

But you have to mute the computer before you shut it down. Remembering to perform an action every single time in order to prevent a startup chime is sub-optimal. Having an option to turn it off would be better than relying on a 3rd party hack.