Turn off Mac boot up sound

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