How do I disable the startup chime on MacOs High Sierra?

[asdfguy]

I have a 2015 15" MacBook Pro running MacOS High Sierra and I have a hard time disabling the startup chime. I have tried the following code and the don't seem to work.

sudo nvram SystemAudioVolume=%01
sudo nvram SystemAudioVolume=%00 Not working
sudo nvram SystemAudioVolume=" "

One of the code -- can't remember which one -- doesn’t play start up chime when I power on the laptop from shutdown, but when I restart, the chime still plays.

Another code -- once again I can't remember which one -- doesn’t play the chime when I restart the laptop, but when I power it on from shutdown, it plays the chime.

The other code doesn’t work at all.

Is there actually a code that fully disables the startup chime completely?

 

[MacRS4]

I use these on my 2017 units - you could try them? I don't have any 2015 units around to test.

Audio
sudo nvram BootAudio=%01

sudo nvram BootAudio=%00

Auto-Boot
sudo nvram AutoBoot=%00

sudo nvram AutoBoot=%03

 

[Feek]

Related but not relevant - I only just realised that my 2017 iMac doesn't make a startup chime.