How to spoof a microphone to a program?

I often need to join Zoom meetings where I am merely a participant (that is, listening only and not speaking).

Zoom lets you join without a webcam fine, but moronically requires you to have an audio input as well as output to use computer audio. This means if you have a speaker/headphones/earbuds, too bad, so sad - according to Zoom - unless you also have a mic connected to your computer.

This means that I need to dial in by telephone even though I have a perfectly functional audio output, which works but is not ideal.

There's probably little chance of getting Zoom to use some common sense here, so I'm wondering how easy it would be to "spoof" the existence of a microphone to Zoom at the application layer from Windows, to make it complacent and let me use computer audio. I don't need the spoofed mic to work, or actually do anything at all, but just be detectable by Zoom so that it'll let me proceed.

Is there a way to do this, without physically connecting any hardware to the computer?

If you googlge "Windows virtual mic", you can find many solutions.

One of them is Virtual Audio Cable, which is donation-ware. Upon install, it will create a virtual input device and a virtual output device. To use the "mic" / input device, you'll have to go to Control Panel -> Hardware and Sound -> Manage audio devices, and in the "Recording" tab enable the "CABLE output" device. (left side of image)

Then, in Zoom, in "Audio Settings", choose "CABLE output" as your microphone. Just ignore the "if you cannot hear your voice..." message. (right side of image).

EDIT: If you've ever connected a docking station or any audio adapter to your PC, chances are good that in the "audio devices" you'll still see the recording devices, but in a "disabled" state (see my image: I have three of them!). It's enough to manually enable them, and Zoom will accept that as a microphone. That way you don't even need to install any software!

I'm not familiar with Zoom, however, some systems will simply accept anything plugged into the mic socket as evidence of a microphone being attached.

Have you tried plugging an old earphone with an appropriate jack plug into that socket?

You are more likely to own an earphone or headphone. A broken one should do, or even just cut off the jack plug and use that.