I have a Mac Mini (Late 2014) and it always DID have a startup sound (right after pressing the power button). Unfortunately that sound is gone since I updated to Big Sur. I checked System Preferences - Sound. But the checkbox for 'play sound on startup' is checked. I even tried unchecking and rechecking it. But to no avail. To disable the startup sound, just press the “Mute” button on your keyboard (that’s the F10 key on a MacBook) before shutting down or 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.

Wow... so obvious. Once you know the answer.
Can't believe I never thought to try that. Thanks.

The startup chime simply plays at whatever volume your computer was set to when it shut down. So rather than mucking around in Sound Preferences, just press the mute button on your keyboard before shutting down.

Not so, Sesquipedalian. On my 2008 Mac Pro running 10.7.4, the startup chime doesn't 'simply' play at the volume you've set. It's true that muting the sound does suppress the startup chime, which I hadn't realized. But this hint was a revelation to me because I ALSO didn't realize I could set volumes for the internal and external speakers separately and have them STICK independently.
My startup chime plays through the internal speakers, and I want it always set at a low volume (but not muted). Other audio plays through my external amplifier and speakers, and I'm constantly adjusting that volume to suit what I'm listening to. Thanks, S.Patton, for the helpful hint!

Have you tried the startup sound pref pane?
http://www5e.biglobe.ne.jp/~arcana/software.en.html#StartupSound
I used it on my Mom's iMac and it worked nicely. I am not sure how it operates in Lion, however.

You could also install the Arcana StartupSound Preference pane, that mutes the startup chime without altering your volume settings (actually it sets volume to zero at shutdown and resets it after boot, but gets the job done). You can find it on Macupdate.
Currently it doesn't work under Lion.
Another simple solution is using two scripts, one running at shutdown and the other at startup, that do the same thing: set volume to zero wen shutting down and reset to custom preferences at login.

Their website (http://www5e.biglobe.ne.jp/~arcana/index.en.html) states the incompatibility with Lion as well. It was a useful utility to me until now. And, I hope that it will be updated to work with Lion and Mountain Lion.

Is this really a hint?

Mac Startup Sound Catalina

Yes! Absolutely it is! It just needs to add one additional step:
5) don't touch the volume ever ever ever again in fact rip the keys off your keyboard because this is a Mac OS X Hints hint.

Is this really a reply?

The hint is: volume control is separate for internal and external speakers. Seems to me that that is rather obvious to anyone who has ever plugged in headphones.

Being reader for a long time, I had to register just to make things clearer about this! (two things to understand):
- Starting chime ALWAYS sounds from the speaker/s of your mac,
- All Apple machines remember independient volume settings; just adjust each (your macios knows the last volume level you chose without a line out connected, so unplug the wire -ex. speakers/earphones- and JUST set volume down, even till mute, if you want. Plug on the sound line out, and JUST choose the volume level. On next startup, chime will sound even muted -as playing trough int. speakers, and every else will sound as plugged)
In all, is a very consistent behavior, and forget going to Preferences; it's not needed.
(I once spent some time searching for an app to mute startup... and it was all much easier!)

Perhaps because I always adjust the balance setting (my Macbook is used closer to one of the extermal speakers, so I can't have it in the middle) my sound settings, including the volume ALWAYS reset on a reboot. The balance goes back to the middle, the volume always increases. Which of course means I have to re-adjust EVERY time I reboot. (which I don't have to do often, but still, it's annoying)

Mac Startup Sound Mp3

I use Psst and it still works in Lion. Much easier and it has volume control. Donation.

Mac Startup Sound Too Loud

Apparently, this hint works for some people. It also doesn't work for many people, as an Internet search will reveal. I've tried this hint on a 2011 MacBook Pro, a 2008 MacBook Pro, a 2009 iMac, and several other computers. I've tried all the variations that I can think of. I set the internal speakers to no volume, with and without the Mute checkbox checked. I've set the external speakers to no volume (Headphone, in the Sound control panel), again, with and without the Mute checkbox checked. With these settings, I've restarted with the external speakers plugged in and with them unplugged. I've restarted with the Mute button on the keyboard pressed. In every case, the chime sounds at its normal loud volume, from the internal speakers, on every restart.
I don't know why this hint works for some people and not for so many others. If anyone can shed light on the reason for the differences, it would be a very valuable addition to this hint.