I have heard of that too, and I declined that therapy. It was described to me thus:
You sit in a hydrotherapy pool, with your back (and kidneys) against the side of the pool. They wheel the sonic cannon (I don't really remember the name of the machine, some kind of ultrasound generator) line it up against the wall opposite your kidneys and turn it on. The desired effect is to have the sound travel through the mostly uniform medium of water and then when it hits something different, like the kidney stone, it would rattle/vibrate/shake/pound it into dust.
Think about it. They are trying to crush a pebble six inches inside your body, in your freaking kidney, by applying a force from outside. How hard do you have to hit it, do you think? I was told that it's like getting pounded with a two by four. That prospect was very unappealing in my current condition. Maybe if the procedure called for hitting me on the head, there would have been a chance...No thanks.
Edit: I looked it up.
Extracorporeal Shockwave Lithotripsy