DEPRECATED
See repository here for a Unity 2019 sample that includes Loopback and Microphone as well as a VFX Graph example:
https://github.com/hallidev/UnityWasapiAudio
A barebones example of WASAPI Loopback Audio in Unity using CSCore
Check out CSCore: https://github.com/filoe/cscore
Instructions:
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 -> 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: Bang that "Play" button
That's it!
You won't see the bar analyzer until you hit Play - it's instantiated at runtime.