On a Chromebook, you might find your mic muted and grayed out while sharing your screen in Zoom. In Zoom on a Chromebook, you can either share their system audio (e.g. to play a video), or their microphone, but not both at the same time. To talk, hit the little "Pause Screenshare Audio" button at the top of the screen to talk, which looks like a speaker:



Also, if you're about to share your screen, and you don't need audio (e.g. from playing a video on your PC), you can untick the "Share audio" box when sharing your screen: