This is a Lightstream Labs feature. For info on enabling Labs features please visit this article
An audio source can be added to capture an external microphone or your computer's audio.
Please be aware that if you're currently capturing a window or display layer, then your audio will automatically be brought in via that layer and there's no need to add another one.
If you're using a webcam you should add your audio source under the "properties" of your webcam as seen here.
To add an Audio source you'll need to add your computer to your Lightstream account using the steps found here.