How do you turn off the start-up sound

You don't, it is part of an important diagnostic. In effect it is saying 'Yipee, I have found an operating system!'

Here is a link to a discussion that suggest ways to silence the chime, and reasons for not silencing the chime:

https://discussions.apple.com/thread/700230

Good luck.

If you don't want the tone to disturb others, you can always leave external speakers (turned OFF) or a headset plugged into the audio output jack. This prevents the internal speaker from playing any sounds.