[snorkel]

Yup, all client side. You can view source on the page and read the javascript code doing the Fourier Transforms on the audio sample.

[kybernetikos]

What's the socket.io for?

Also, am I right in saying that you did your own fft rather than use the built in analyser? I tried to do one with the built in analyser a while back and had mixed results.

[nanidin]

He's using (or at least loading) the DPS.js library: https://github.com/corbanbrook/dsp.js/