Hello,
I have this issue as well. What it is... The microphone wire within the camera becomes loose over time. This is due to the turning and spinning of the camera as you pan the room. You should be able to spin it until it catches the wire, bringing sound back to the microphone. In my case, if I turn the camera to face the left upper corning of the room, I will regain sound. I can then move it back to the baby, and keep sound until my wife moves the camera.... after I tell her not too.