No Voice Recording for Incoming Video on QuickTime On Mac
In this case you can't capture 2 recordings simultaneously with Quicktime. You have use additional Apps e.g Garage Band, Audacity, Audio Hijack or a iPhone to record your internal audio. 

In previous Quicktime Pro Versions it was possible insert a audio sequence to a Quicktime Movie.

Here’s a quick how-to:


Open Apple iMovie
Import your Google Hangout Video Screen Capture 
Add your audio recording to your iMovie project.
Export your Video