how do I restore the boot up sound

mrbillyt wrote:

How do I restore the boot up sound on my iMac 5K circa 2017 running Mojave?

You can not. Sorry.

About Mac startup tones

https://support.apple.com/en-us/HT202768

Apple Feedback http://www.apple.com/feedback

My old Mac mini 2009 once lost its startup bong but that could be fixed by resetting PRAM via Option-Command-P-R boot.

But Mac models built after late 2016 do not have this sound effect on boot, except for the 2017 MacBook Air.

http://osxdaily.com/2017/07/14/where-startup-sound-new-imac-macbook-pro/