Listening to Sound Card Output

Hi, I'm working on a sound to (ingame)light tool for Unity. I'd like to be able to analyze a song that is playing in e.g. Windows Media Player or similar software in real time and make my ingame lights dance to it.

I'm already getting quite interesting results using a microphone, by recording microphone input into an AudioClip, playing that clip in a muted AudioSource and then using AudioSource.GetSpectrumData in order to do some beat recognition etc. - inspired by this tutorial.

Would it be possible to do something similar using the current output of the sound card rather than microphone input? The ideal case would be if I could still use AudioSource.GetSpectrumData or a similar method.