Here's how you can turn off the sound your Mac makes when it boots up. 1. Open a terminal window. 2. Type in the following command, then hit Enter. sudo nvram SystemAudioVolume=%80 3. Type in your password. 4. Restart your Mac, the boot up sound will be off. 5. You can turn the sound back on by using this command. sudo nvram -d SystemAudioVolume
For more, see the original article at the link below.
Silence Your Mac Boot Startup Sound Altogether | Cult of Mac