After you click the "New Screen Recording" button a small rectangular window appears. Next to the Record button there is a drop down menu where you can indicat

After you click the "New Screen Recording" button a small rectangular window appears. Next to the Record button there is a drop down menu where you can indicate sound input source, as well as things like "Show Mouse Clicks". I recently came across this Quicktime tool. It's really nice, you can even do some simple movie editing with it (trim clips, combine clips etc). I use it to produce demo videos.