I saw one post where the user said that dust got in the Mic hole. Vacuumed it and restored the audio.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime