Click on the top right toggle next to the webcam icon, to enable audio/video. Be sure to choose "allow" on any pop-ups from your browser, to allow for the device to be used.
On the "My Webcam" window, click the green "ON" button next to the webcam, to turn it off. You will now be in audio-only mode and users will see a microphone next to your name instead of a webcam.