Mac startup sound keeps coming back to haunt me
You could try using a free application called Deeper by Titanium Software. You can download it from this link here, or by using homebrew cask in which you can enter the following terminal command after installing homebrew/homebrew cask:

brew cask install deeper


Once you have installed deeper and allowed it proper privilages, open it up and go to the "Login Tab". From there you will see a button to turn off your mac's startup sound. Click it and you should be good to go.