Google Meet microphone problems are almost always caused by a browser permission block or the wrong mic selected. Click the padlock icon in Chrome's address bar and set Microphone to Allow for meet.google.com, then reload the page. In Meet, click the three dots > Settings > Audio and pick your microphone by name instead of Default. If Meet shows "Your microphone is muted by system settings," check your OS: on Windows go to Settings > Privacy & Security > Microphone; on Mac go to System Settings > Privacy & Security > Microphone. Make sure Chrome has access. Close other apps using the mic and restart the browser after changing any permissions.