Allow microphone

I am part of the way there. Using Mojave on a 2017 iMac, when I go to https://askapple.apple.com/Chat/ , I see the following:

If I click "Allow" I see "A Specialist will be with you shortly".

The problem is that I don't then see the setting in Safari Websites preferences, though in Sonoma, on a different computer, I see:

Safari Websites does not save the choice in Catalina either, the furthest you can update, though it also gives "A Specialist will be with you shortly." None of these seem to save to Safari Websites preferences -> Microphone (Blank for me on all OS versions). At the bottom of this on the right, you see "When visiting other websites - Ask". I am wary of suggesting toggling it to Allow for the time spent on the Apple site, but it might be an idea. In general, I would be inclined to enable only temporarily microphone access to any website. I will keep thinking and report back, but there are quite a few sources of advice on your Mac update, and I am sure it will move you to a different world.

First thing to check is System Preferences -> Sound -> Input that looks something like this in Mojave, and make sure that the Input volume is not set to zero:

I think you may be looking at Microphone in Security & Privacy, where you only find applications and cannot add anything yourself. From personal experience, another thing to think about is whether your iMac has ever been taken apart. The only time I lost the microphone on an iMac was when an Apple Authorized Service Provider took it apart under warranty to replace a component. The engineer later showed me, with profuse apologies, how he had inadvertently trapped the screen seal over the microphone, putting it out of action.

Not sure I can answer, as I have just looked through the three Macs (five operating systems) that I have to hand and do not find any web sites. If you could point me to the support page that causes the problem, I will check with the Mac that still runs Mojave and see how it works. Just a thought, is your current iMac compatible with Mojave? See macOS Mojave Technical Specifications Beyond that, another source suggests that a 2019 iMac would run it too but, in general, they don't run an OS earlier than the one originally installed.