I understand the sound data are generated by JavaScript. How many distinct sounds are generated? How much space is required to store this data? Are the notes of a chord combined by JavaScript or the browser?
If you click the square in the upper right, you'll get options to change the waveform and volume response. There are some other options too, like shifting by an octave or changing the color of the piano :)
Better things could be done by using the web audio APIs, but I explicitly started this project wanting to build audio using data URIs.