TIP: Soundflower WORKS in Big Sur
I was expecting to have to say goodbye to this old extension, but I'm pleased to confirm that it still works.
Soundflower provides additional audio outputs and inputs, allowing you to reroute audio output from one application as input to another.
You have to use the most recent version, 2.0b2, and curiously the installer says it "failed", but in fact the extension is installed in /Library/Extensions, and upon restart everything is working fine.
Just I used to do before, I created a Multi-output device in Audio MIDI Setup, with the speakers and Soundflower (so I can monitor the sound as it is playing), and was able to record using Quicktime Player.
Works perfectly.
HTH