Probably related to a wifi disconnection problem.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime: Set-up, Use, and Troubleshooting Problems
I saw one post where a user said dust got inside the small microphone hole. Using a vacuum cleaner removed the dust and restored the audio.