One feature that many users miss on more recent Mac models is the startup chime. Although once a plague in labs and classrooms, there’s something deeply emotive about that chord.

For some time, this could be re-enabled on muted models by setting the BootAudio parameter to 01, but that stopped working in models released from about 2017. Thanks to @waly_k for discovering the new secret way to enable this, which works even with the iMac Pro and MacBook Pro 2019 16-inch. This sets the StartupMute parameter instead.

To turn on your missing startup chime, open Terminal and type
sudo nvram StartupMute=%00
then press Return. You’ll be prompted for your admin user password, which you should type in and press Return again. Then close Terminal, and try restarting your Mac.

You should now hear that warm, reassuring chord every time your Mac starts up, until you next reset your NVRAM, after which you’ll need to repeat the process of turning it back on.

Happiness is.

Postscript

To turn off the startup chime, open Terminal and type
sudo nvram StartupMute=%01

Thank you to Michael Bach (comment below) for reminding me to point that out, although I can’t think why anyone would ever want to do it 🙂

The easiest way to mute the chime when you need to is to put a blank audio jack plug into the headphone socket, where your Mac has such a socket. That should trick your Mac into thinking that headphones are connected, and mute its internal speakers. If you have a whole roomful of Macs, it’s probably better to turn the chime off in the NVRAM, but in the past some sysadmins resorted to gluing audio jacks into headphone sockets to stop users from enabling the startup chime.

If you want to be able to change the volume of the chime easily on models with headphone sockets, connect to an external audio system and use its main volume control, setting it low before shutting down or restarting, for example.

(Thanks to @XwCypher for pointing out that the BootAudio parameter has inverse logic to StartupMute.)