Screencapture with Audio using QuickTime on OSX

You need to install Soundflower:

https://rogueamoeba.com/freebies/soundflower/

In the QuickTime Player window, click the down arrow to the right of the record button and select "Soundflower (2ch)" as your input. Then launch System Preferences and go to the Sound settings. Go to the Output tab and select the same "Soundflower (2ch)" option as you did in QuickTime Player.

You can download it from here:

https://github.com/mattingalls/Soundflower/releases

Here's the direct link:

https://github.com/mattingalls/Soundflower/releases/download/2.0b2/Soundflower-2.0b2.dmg