It looks like that GitHub repository doesn’t have any substantial code. In the presentation he mentioned it uses a special version of MakeCode (around https://youtu.be/LzkN4j2DB_w?t=2032). The video is too blurry to see the URL.
Thanks Bill for looking into the code behind. I will check with Joshua and get back with hopefully good news as I think that would make the microphone much more interesting & with an extension easy to use.