I like the idea but I think it would be more useful it it was more reliable. Ignore all types of speed requirements and make it super reliable then it could actually be useful in special circumstances.

Receiving the audible sound did not work at all in any browser.

Receiving the ultrasonic (that is not very ultrasonic at all clearly hearable) did work in chrome and firefox.

Sometimes it figured out that there had been a message but not what it was. (saying missing packets)

Sending worked from all browsers.