How to stop startup chime on boot up
This is pretty easy. In fact there are articles out there that describe how to do this.
Briefly, open terminal app in /Applications/Utilities and paste in this command
sudo nvram StartupMute=%01
It will ask for your password and then just return to the terminal prompt. When you reboot the chime will be gone.
Note that if you reset NVRAM (some old farts like me still call it PRAM) the chime will return, you just have to do that command again.
to turn the chime on
sudo nvram StartupMute=%00
In case you change your mind...