If your mic isn't working
Windows
Open Settings, then System, then Sound. Under Input, choose your microphone and check the volume. Under Privacy & security, then Microphone, make sure apps and desktop apps are allowed.
Mac
Open System Settings, then Sound, then Input, and choose your microphone. Under Privacy & Security, then Microphone, turn on your browser.
iPhone and Android
Allow the microphone for this site when your browser asks. If you blocked it, change it in the browser's site settings, then reload.