This happens most often on a computer.  Occasionally upon startup, you may see a window that says your microphone is muted or that is is not connected. The most common error errors are as follows:

• The microphone recording volume in the computer's sound settings is set too low. Raise the recording volume to 75%.
• The microphone was plugged in after the start-up process. Refresh the page in the browser
• The mute button on the USB controller is engaged.  Check the toggle switch on the USB controls.

Another cure for “muted microphone” is to redo the microphone settings. Be sure that you have clicked “allow” if the browser requests access to the microphone.