You can mute your own audio at any point during the call by clicking on the 
button. Your audio channel will be turned off and the other participant will no longer be able to hear you.
The mute button is found during a call on the In-Call Action Buttons which are displayed beneath your video stream once the call is initiated.
Once you click the mute button, it will show as 
clicking on it again will remove the strikethrough and reconnect your audio stream to the call.