We've seen artists paint with light in urban landscapes, but not so much with sound. Highly directional sound emitters have been used in exhibits, like Holosonic’s Audio Spotlights were at the Epcot Center years ago (http://www.holosonics.com/PR_Epcot.html), but I haven’t yet seen attempts at using sound to paint a space with such devices.
I would be interested in seeing if we can incorporate Audio Spotlights with haptic interfaces to enable people to collude in generating an auditory atmosphere in an urban space.
One plausible technique would be to install motion detectors in movable boxes, chairs, planters, or crates in a well trafficked space. The detectors, depending upon the movement of the boxes relative to their original placement, could wirelessly broadcast their change in position to a PC, which in turn would modulate the sounds/music projected into the space.
I believe that equipment similar to Audio Spotlights is rentable.