every time when mac os boots or startups from shutdown situation,

the mac os will play a loud "pong" sound as reminder,

you can temperally mute this sound by pressing the F10 function button,

however, in order to terminate it for good,

I tried several ways, finally this one did the work,

here it is,

first of all, start a terminal window when you login to mac os,

try press "command+space", to use spotlight search,

in your terminal window, make two bash files,

$ sudo nano /Library/Scripts/sound-off.sh

inside the sound-off.sh, save these contents:

#!/bin/bash

osascript -e 'set volume output muted 1'

###

and save those to sound-on.sh in the same directory, which is "/Library/Scripts/"

#!/bin/bash

osascript -e 'set volume 4'

###

then make these two scripts have the privilege to execute by using "chmod"

$ sudo chmod u+x /Library/Scripts/sound-on.sh

$ sudo chmod u+x /Library/Scripts/sound-off.sh

at last, make these actions to active in mac os by using:

$ sudo defaults write com.apple.loginwindow LogoutHook /Library/Scripts/sound-off.sh

$ sudo defaults write com.apple.loginwindow LoginHook /Library/Scripts/sound-on.sh

these steps did work on mac os mojave 10.14.2

or you can just download a free app from App store called "Silent Start"

if you decided to use Silent Start, you can still undo the write defaults above by execute these commands:

$ sudo defaults delete com.apple.loginwindow LogoutHook

$ sudo defaults delete com.apple.loginwindow LoginHook