Turn off Mac boot up sound
How to disable your Mac's startup sound in OS X
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