When I designed a lock/unlock button for my app, I added an arrow to indicate what action occurs when you click the button: https://www.youtube.com/watch?v=9MZmwkn-2rw&t=117s

For an unmute button, you could have a microphone with a line through it, and an arrowhead indicating that clicking the button will remove the line. Then the mute button could show a microphone with an arrowhead pointing inward from the corner, indicating that a line will be drawn.