I recently came across a system preference that allows you to mute your Mac’s startup sound, or set the volume lower if you prefer.
StartupSound.prefPane is the software which controls the volume of the startup sound of your Macintosh computer. StartupSound.prefPane mute the startup sound without changing the output volume setting. And StartupSound.prefPane limits the startup volume so that it may not become loud even if you make the output volume louder. To set up the startup volume, you select the “Startup Sound” pane added to the “System Preferences.”
Works like a charm, is free and can be found here: