My understanding is that a Snowden-leaked 2008 NSA internal catalog contains airgap-hopping exploits by the dozen, and that the existence of successful attacks on air gapped networks (like Stuxnet) are documented and not controversial.

This understanding comes in large measure from a casual reading of Bruce Schneier's blog. I am not an security expert and my "you don't understand what you're talking about" reflexes are firing.

But moving to areas where I know more, I think e.g. if I tried writing a program to take as input the sounds of someone typing and output the letters they typed, I'd have a decent chance of success.