Click the button that says Use audio input and then click Allow when prompted to give your browser access to your computer's audio input. You can then click Use audio file to switch back to the audio file.
I've also added an intensity control that allows you to juice up the visual intensity without sacrificing volume or accuracy.
Of course, it may not work in your browser:
Is it ever that simple? At time of writing HTML5 live audio input is only supported in Chrome Canary. You need to go to about:flags in the URL bar, and then enable Web Audio Input near the bottom of the list
Spectrum Analyzer on github