Connect your LeapMotion, launch your browser, point to the index.html. Hold a few fingers over your LeapMotion and move them as if you were hitting a keyboard (or, more appropriately, playing a piano).
The sounds are generated via Data URIs only. No audio files were harmed in the process of making this demo. See http://mrcoles.com/piano/ - this is where I got the idea (and the code for audio.js) from.