Question - could you just send ultrasound though the walls just like the wifi signals in this example? I guess I'm not sure how much more limited ultrasound would be in this case.
It doesn't go through walls, so they are pretty much the right tools for the job. They are known vulnerable to lasers, but well microphones themselves are, so...
No. Ultrasound will not make it to the other side of a wall, especially not an outside wall.
Infrasound could, but the speakers might not be big enough, unless you use the amplitude trick.