It doesn't sync to live audio it records/grabs. It uses the Spotify API that provides rythmn/intensity mappings for most (all?) songs. So basically the web page asks the API which song you're currently playing on Spotify and what BPM it has and it renders the visualization accordingly.

Also:

> In the future I'm going to tap into Spotify listening history so I can surface music recommendations and other features.

See: https://github.com/zachwinter/kaleidosync/blob/6136f79988867...