By default, QuickTime will not allow for system audio recording-- only microphone. System audio can be routed with the Soundflower driver. Once Soundflower is available on the system, it can be used natively after setting up a custom Multi-Output source.
- Install the Soundflower kext (project found on GitHub)
- Use Audio MIDI Setup app to create a new Multi-Output Device
- Set the Built-in Output and Soundflower (2ch) as sources within that new device
- Switch to the new Multi-Output Device in System Preferences
- QuickTime can now record system audio -- new recording >> select the Soundflower (2ch) from the source list next to the record button