High Sierra Startup Chime / Bong Missing

There is not. Lots of users upset by its removal. I don't know why it was removed. All you can do is register your complaint at the link I provided. Then hope enough users have done the same which may convince Apple to put it back.

The secret, I discovered, is to disconnect any externally connect audio devices that use the Headphone jack so audio is connected to the internal speakers. Then use the Sound panel to adjust the sound to your desired level. This should result in a quieter chime because the chime plays through the internal speakers. I've "heard" it work! 😀

On the first model that had the chime missing (2016 MacBook Pro), it could be re-enabled with a terminal command, according to a blog. Apparently it was initially disabled instead of removed. That likely doesn’t apply to all subsequent Mac models.

I know. It wasn't suggested as a solution. It was simply information for you. I did not know you only have an iMac Pro. Not too many people here need or can afford one. Obviously, you're a fat-cat. 😁😁

Nope. Apple removed the chime. Sorry. You can complain at Feedback - Apple Support.

FWIW, there were also lots of complaints it was too loud, couldn't turn it down or off, etc. No one is ever happy.

That's interesting because with the sound effects volume maxed out and the output volume set to match on my iMac Pro I get zip on start up. 😎

I don't think the secret works on the new Macs without the chime. Works for all of my not new models. 😎

I'm sure it does. The OP seems to have a 2017 iMac. 😎

Oh. Actually I have six Macs and the chime never bothered me. Plus I know how disable it or turn it down etc. 🙂

Thanks though. My original comment was just about why Apple may have removed it. 😎