This is useful for recording Let's Plays of Video Games, Skype Calls, and other projects that require the output audio as well as an additional input device.
Soundflower link: https://github.com/mattingalls/Soundf...
After installing Soundflower you will need to go into Audio MIDI Setup, create a Multi-Output Device, so that you can link all output audio to both your main output and Soundflower, and set the new device as the default device. Then set the input device on the capture software to Soundflower.
To record both your voice (or other input device) along with Soundflower, you will need to create an aggregate device, which will allow the capture software to record multiple inputs at once. Garageband doesn't support recording of more than two channels (stereo), and since this aggregate device has four, you cannot use Garageband for your project. However, if you record using QuickTime Player's audio recorder, it'll keep the four channels separately in the same file, and if you install Audacity and import the recording into an Audacity project, you have control over all four channels individually. You can then export the recording as a stereo audio file.
If you want to record voice and audio output while recording your screen, OBS, or Open Broadcasting Software, is a great tool for that. It lets you add multiple input devices into the same stereo mix. Not only is OBS a great streaming program, it's recording is excellent too.
Full disclaimer: Recording Skype calls may be unlawful in your area. You must inform every participant in your party that you are recording. I am not responsible of any damages to you or your property, nor am I responsible for any legal troubles you may get into.