There is two emitters, stereo speakers, you know their offset in relation to the receiver, the mic, so you first wait for the first pulse in the respective frequencies of both speakers (you can do it in parallel as long as each speaker use a different frequency or set of frequencies) that is comming directly from the speakers and any pulse after the first one for each speaker will be echoes, the time difference between the first and the echoes is the distance from the respective emitter: 2 distances from two points you know the coordinate with a bit of trig gives you the 2d coordinates.