Hello @edspark,
I am having the same problem you are. In my case, I use an external microphone on my Mac and cannot figure out how to get it to use that Microphone.
However, you can make an audio recording in something else, like Quicktime (which allows you to select the microphone), and then upload the audio file:

I have asked about the microphone selection issue and will reply if I get a solution.
droopy