To control the audio select Audio from the bottom right.
When you have clicked Audio a popup window opens up which is called Audio Control Panel.
Here are the available audio channel shown.
In the example only the LiveSquawk is available with a green dot, which means it is turned on.
By clicking on the dot it will turn red and will be muted. You can also adjust the volume level by dragging the dot in the Volume column left or right and in the example shown at 50.