First of all, the member must have activated their webcam in one of the boxes, and they must have activated their microphone.
By default, you won't hear the sound from a member who has their mic on until you manually activate it.
Drag your cursor over their box and click on the mic icon. The dot will go from red to green to indicate that the sound is on.
To turn off the sound, click again on the mic icon. The dot will go from green to red to indicate that the sound is off.