If you have a slow connection, or just want to talk to someone without the video part of FaceTime, you can simply hide the FaceTime app on your Mac (Command+H or FaceTime, Hide FaceTime). This will pause the video signal between you and the other party, but leave audio on. This uses less bandwidth and processing power. You can quickly switch back to video by unhiding the app.
Mac Video Tutorials and Tips for Apple Users
