Very small button bottom right on back of iPad,next to sound switch bar. switch it from red to a closed position, so you can't see the red light. This will solve problem. It also explains why it worked previously, as you may have switched it accidentally when adjusting your sound!
I had the same problem. You have to make sure the tiny button next to your volume control on your ipad is showing white and not red. I had mine on red to freeze my orientation and I had no sound using skype. When I flipped it to white the sound was fine.