Soundflower is an open source kernel extension for MacOS, designed to create a virtual audio output device that can also act as an input.
Audio Hijack Alternative
Rogue Amoeba is home to fantastic MacOS audio products for consumers and professionals alike. Rogue Amoeba - Strange Name. Record any audio, with Audio Hijack! Save audio from applications like iTunes, Skype, FaceTime, VoiceOver, Safari, or from the web or hardware devices like microphones and mixers. Record in several audio formats including lossless formats, start/stop recordings at specific times, and even split files while recording based on a pre-determined size.
Initial development and maintenance of Soundflower was done by Cycling '74. In 2014, Cycling '74 passed stewardship of Soundflower to Rogue Amoeba, but we ultimately didn't have the time or resources to improve the product.
Thankfully, at the end of 2015, Soundflower's original author Matt Ingalls picked up the baton. The project can now be found via this GitHub link. If you're looking to use Soundflower, that's the link to visit.
If you've previously used Soundflower, or are simply looking for a reliable tool to route audio between applications on your Mac, you may be interested in Loopback.
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Loopback
Loopback is a modern, fully-featured virtual audio device tool of our own. With a robust infrastructure and full support, it's the ideal tool to route audio between applications and more.
Learn more on the Loopback page.