Here's a brief timelapse video documenting a portion of the user tests for both my intro vignette and my sonified train schedule vignette. In the latter, users hear the trains arriving from different directions and look around to hear a sped-up timelapse of Grand Central's morning rush hour schedule set to music.
The tests were really illuminating. Primarily, it was clear that sound design elements work better at slow tempos. For the 3D head tracking feature to be recognizable to users when there is no visual display to indicate where a sound source is located in the virtual geography, it must ring out with a long enough sustain that someone can look towards it and locate it.