My family lives in Arizona, and I use FaceTime fairly regularly to stay in touch with them. Occasionally though, I’ve run into issues either with their network or mine where we’d get erratic video and poor audio.
In a moment of frustration during a recent call, we discovered that FaceTime will continue to function as voice only on an iDevice if you bring yourself back to the home screen. Make your call as normal, and then once you’re connected, either press the home button to bring yourself back to your main screen or press the power button to put the device to sleep. The audio call will continue and you should notice a quality increase.
In my case, I can continue to use bandwidth hogging applications (any other MMO players out there?) and still talk to my family.