How can unity hear the sounds being played on my speakers?

I want to create a game where there will be an audio spectrum visualizer, which changes heights/colors according to the sound being played on the device’s speakers.

What should I do? Should I:

  1. Use the Microphone class referring to the speakers? If so, how?
  2. Should I use a url, through the www class?
  3. Or is there a simpler way?

I think you are looking for this YouTube tutorial