An example of WASAPI Loopback Audio in Unity using CSCore
Check out CSCore: https://awesomeopensource.com/project/filoe/cscore
This is an updated version of my little Unity audio visualization library including a VFX Graph example.
Instructions (Unity 2020.x, HDRP):
1: Clone / download
2: Open in Unity
3: IMPORTANT! If you are going to be making a build using this, you need to go to Edit -> Project Settings -> Player -> Other Settings -> Api Compatibility Level and set it to ".NET 4.x". Otherwise you will get a kernel exception upon trying to run Unity Player.
4: Play some music on your computer
5: Hit "Play"
If you don't have VFX Graph installed and would like to use this, just copy the WasapiAudio folder into your own assets folder and delete the Vfx.* files that won't build. It should work going back to Unity 2017.x at least.