What gave you the idea for the game? I was really impressed with the idea of just listening to someone's voice and avoiding the directions that you heard tones in. Technologically it reminds me of the Glide Slope Indicator technology used on airfileds to land airplanes. In that technology two radio frequencies are projected down a runway and the landing airplane has to fly between them. The pilots don't hear the frequencies, but the plane's instruments turn the signals into visual indicators that tell the pilots how far right or left of the runway center line they have drifted.
That's interesting, thanks - I hadn't made that connection! The original idea for the game mechanic was active echolocation (sonar), which comes in a bit later, but I thought I needed some sort of compass to guide you through in general (otherwise it was a bit too hard), and after a few experiments the tones turned out best. The "no graphics" constraint itself was something I'd been toying with & have wanted to try for a while.