I just joined the Forum and submitted the following message but to the wrong thread. Here is that message, plus an important addition at the bottom:
just upgraded my OS from 10.13 to 10.15 and the mic input did not work for Audacity.
After reading posts on this and learning about 10.15, I found the solution.
(1) Go to System Preferences / Security and Privacy
(2) choose Privacy in the top nav tab
(3) After making sure you have permission (unlock), click on Microphone in the left column
(4) Allow Audacity to have access.
Apparently, the default for Apple is not to have access for the microphone to applications. The Audacity folks should make it very clear in their notes that it is necessary to do something like what I have done. I am now to Catalina, and have not figured out when I want to give applications/providers access to my computer. I am attaching the last step in the process I describe above. Apparently Zoom is allowed access automatically to the microphone, as I did not set it consciously and recorded a zoom session since I upgraded to 10.15.