Earlier quoted context omitted.
sudo nvram SystemAudioVolume=0 If this works for you, please donate the $10 to Archive.org.
If that works for me on my Mac Mini, I'll donate $10 to Archive.org too.
Actually, it worked even better than I expected - it still chimes, but almost inaudibly, which means it won't wake anyone up but I can still tell I've hit the power switch - so $10 duly donated. Thanks v. much.