How to record both screen and sound with Quicktime on El Capitan?
You  need to install Soundflower in order to run it on El Capitan. El Capitan requires kext to be signed in order to load them. This one gets its kext installed in /Library/Extensions/.

This is due to System Integrity Protection

Then, you have to create a multi-output device with: Audio MIDI Setup.app, which is found in /Applications/Utilities/ :



Finally, when you want to do the actual recording, make sure you use this multi-output device, and capture from the same Soundflower device used in this multi-output device. Otherwise, you can't both listen to and capture the sound, because it goes directly to soundflower without being copied to the built-in output.

alt/option + right clicking on volume gives you this menu:



and Quicktime now looks like this: