How to Disable the Mac Boot Chime Completely
If you’re a Mac owner you know that every time the Mac reboots or starts up it makes a startup chime sound. Though you can temporarily mute the chime by holding down the Mute key on any Mac keyboard, you can also choose to disable it completely by turning to the command line.
Launch Terminal, found in /Applications/Utilities/, then enter the following command to disable the boot chime:
sudo nvram SystemAudioVolume=%80
Enter the admin password when requested, which is required by usage of the sudo command. On your next reboot the Mac will be completely silent. Adjusting the parameter at the end of that command lets you change the total volume of that boot chime, but it appears to favor ascii characters.
To return to the default setting, you can remove the variable with the -d flag.
sudo nvram -d SystemAudioVolume
If you’re not comfortable with making tweaks to OS X with the Terminal, you’ll be better off using the one-off Mute key approach or using a simple tool like StartUp Ninja to silence the sound. StartupNinja is basically just a GUI frontend to the nvram tool discussed here.
Thanks to AnserMan for the tip left in our comments, though they suggested using %00 rather than %80 as the parameter