On Lounge: -
If your mic button has turned red in color with a slash, then you are muted. You can mute and unmute yourself.
On Live Event:-
The attendee's mic and the camera will be muted by default once the session goes live. You will not be able to see yourself but the contents of the live screen. You will be able to see your camera and mic if you are invited to the stage or got in there using the raise hand feature.
On the live stage, the host can mute the speakers and invited attendees but cannot unmute them.