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:
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.
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