MacBookPro Disable startup chime

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Source: https://apple.stackexchange.com/questions/458195/how-to-disable-startup-chime-on-macbook-pro-5-2-2009

`OK after spending countless hours on this, I finally figured it out. And in the end, I didn't even need Mac OS X for it.

Boot into Ubuntu, open a terminal window and issue the following commands:

sudo rm /sys/firmware/efi/efivars/SystemAudioVolume-7c436110-ab2a-4bbb-a880-fe41995c9f82
printf "\x07\x00\x00\x00\x00" > ~/SystemAudioVolume-7c436110-ab2a-4bbb-a880-fe41995c9f82
sudo cp ~/SystemAudioVolume-7c436110-ab2a-4bbb-a880-fe41995c9f82 /sys/firmware/efi/efivars
And yes, deleting and then creating and copying the var back to /sys/efi/efivars was the ONLY method that worked. Attempting to write 
the variable directly would give me a permission denied error, even with sudo and with its' immutable flag having been removed with 
chattr -i.

I'm guessing that you simply cannot modify the SystemAudioVolume var while booted from the Mac OS X installer disk. Whatever value I 
wrote into it with /usr/sbin/nvram from the installer's terminal would always be reset after a reboot.`