Soundflower does work, but you have to do a few things.
- select in the menu bar that sound out must be Soundflower (2ch)
- press shift + cmd + 5
- select video (full screen or area)
- press settings and select Soundflower (2ch) as microphone
- start recording
- after recording, switch audio back to built-in speakers before playback.
You cannot hear the sound while recording, but it can easily be solved by running "Audio MIDI Setup.app" in Utilities and create a multi-output device where you choose built-in speakers and Soundflower (2ch). Then select this as output in the menu-bar. Note that the volume keys do not work as long as the sound is output via multi-output device, so set the desired volume beforehand.