(habe ich nicht getestet, nur gerade rausgesucht)

https://apple.stackexchange.com/questions/221980/os-x-route-audio-output-to-audio-input

If you want to hear the audio while it's playing on your normal Output device (e.g. your Built-in speakers or attached headphones), you need to open the native OS X application Audio Midi Setup. It's located in your Applications folder under /Applications/Utilities.

Press the + button in the bottom left corner, and press Create Multi-Output Device. Mark to use both your Built-in Output and Soundflower (2ch). Now you can hear audio as usual while routing the Output into your Input via Soundflower.

Now that you have created a Multi-Output Device, go back to System Preferences > Sound and select it on the Output tab as in Step 2.