Input microphone not working

I took it to the Apple Genius Bar. They found a piece of tape over the tiny hole at the top of the back of the computer, the input microphone.

{They only found that after full time-consuming diagnostic probing that required totally re-loading the operating system and all of my login material...via Time Machine, which thankfully worked!}.

It's a good example of delving into the electronics and forgetting about simple physical things.

I used Spotlight to search for 'Midi Setup Assistant' which took me to: system_profile.txt. I then searched for 'setup assistant' and found eight entries, none of them related to sound or microphone.

No, nothing like Sound Flower installed.

I had had no problem for years using the input microphone. It was two weeks ago when I tried to enter a conference call and could only hear the participants...but they could not hear me. I don't recall any changes made before that. I've used Zoom and Skype ... they could not have any influence, could they?

The only peripheral plugged in is to an external hard drive (partitioned back-up of Time Machine and Super Duper). I have unplugged that. It makes no difference in System Preference/Sound "Internal microphone/input"

I deleted Zoom, restarted, and deleted Skype and restarted. No effect. Here's Audio MIDI setup.app

I don't have any plug that fits that jack. I only have headphones with the iPhone plug.