how do i turn off the startup sound on macbook pro?

You don't. You may find some third-party software that will, however. Try Auto Mute.

Alternatively, you can try this:

I think this is the best and simplest solution. Open a Terminal window and type (or copy/paste) the following to turn down the startup noise:

sudo nvram SystemAudioVolume="%00"

Press RETURN.

And to restore it to the original value type:

sudo nvram -d SystemAudioVolume

Press RETURN.

That's all folks.

Kappy has a great solution, but do keep in mind that the startup chime is a diagnostic tool so it is not the best thing to turn off.

Confession: I found it by Googling. 😉

Apple doesn't encourage you turning off the beep sound

Here is why:

https://discussions.apple.com/thread/3450133

There is one Solution i know

go to http://homepage1.nifty.com/macbs/download.htm#MuteCon

mutec202.dmg(90.3KB)(recent version)

download site is in japanese but the app is english

works with all versions from snow leopard

you can also add to Admin User acount to launch on startup.

sample screenshot

Kappy... Thank You!

Is there a way to lower the start up chime to about 10%? Maybe just change the percentage value?? "%10" ?

You can just turn down the volume by going to Systems Preferences > Sound and lower the volume slider…there is not a separate control for only the startup chime.