If you are joined in listen only mode you will not have mute/unmute capabilities.
When you have joined the audio after choosing Microphone, the Mute/Unmute Microphone  button appears.
Select this button to turn on/turn off your microphone. Each time you select this button, you will be notified what state your microphone is in; muted or un-muted.
To leave the audio, select Hangup . The icon will revert to the join audio button and clicking it again will display the Audio Chooser dialog.