Its not 100% clear on their site, but I don't think the audio is live triggered. It 's just overlaid on the video documentation of them drawing.
INKSPHERE from INKSPHERE on Vimeo.
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Nice drawing hack, with black ink, for up to two people simultaneously. Second project has fashion retailer 'Nordstrum' allowing passersby to draw in the display window background, using their fingers on the window glass.