Stop the sound when starting up my macbook pro ?

Hi,

Turn the Mac startup sound on or off – Apple Support

Some older Mac OS X systems, even after their system saw an upgrade

to later version, remain limited 'on/off' user-control over startup Chime.

Try the following to Mute, on restart.

Per restart, works just once each time.

[Should work for portable+desktop Mac].

"To disable the startup sound, just press the “Mute” button on your

keyboard (that’s the F10 key on a MacBook) before shutting down

/restarting your Mac. If you ever have to restart your Mac for some

reason but you don’t want it making any noise, just do this."

"If you don’t have a Mute key on your keyboard, you can always just set

the volume slider to the lowest level it will go. That accomplishes the

same thing." (There's also terminal commands that may work, too.)

from: "How to Disable the Boot Sound (or Startup Chime) on a Mac."