The sound on my FaceTime call suddenly became too faint to hear the other voice. All calls have since had this 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

http://tinyurl.com/32drz3d

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.

 Cheers, Tom 😉

Did you try to turn the volume up during FaceTime, the + button at the side.