Disable start sound

that sound is the startup chime that apple use it long time ago and remove it on one update but some people really like it so apple put that back on the Big Sur update, but it's very useful, I use it to turn on recovery mode when it starts playing then I press the key, if there is no startup chime I wouldn't know the right time to press the key.

Apple menu > System Preferences > Sound > Sound Effects uncheck Play Sound On Start Up:

MtD

There WAS ALWAYS a startup sound from the 80's until 4 years ago. I am glad it's back. Apparently an overwhelming amount of Mac users prefer it (I was one of those who "complained") which is why Apple brought it back. It can also be helpful when trying to perform resets which require precise synchronization with start up and keys.

That being said: Got to: System Preferences, Sound Effects, then Un-check "Play sound on startup"