If your microphone is not activating it is usually one of two things:
1) you haven't allowed the microphone in the app or
2) the application is conflicting with another video or meeting app.
You can click the little lock symbol to the left and scroll down to site settings and the to the right of microphone and select allow. Then reload the page and proceed.If that doesn't solve the problem, completely restart your computer and open a Chrome browser and proceed from there.