Unless you happen to take your laptop to a local coffee shop or library that has an infected computer near by. Or someone sets a tiny computer just outside your computer room at your house. YOU ARE NEVER SAFE!
Well, no. Not unless they infect you via some other method. What they've demonstrated is that two computers that are both already infected can communicate via soundwaves we can't hear.
They haven't demonstrated one computer infecting another computer using sound waves.
You have to infect the computer using some other method (and there are plenty, so it's not that big a hurdle) before the sound communication comes into play. The sound communication could allow further attacks, or simpify stealing data from a non internet connected computer.
But you hit the nail on the head about the purpose of such technology - there are many secure computer systems which are not connected to public networks, which reside in the same room as computers that are connected. If they can infect the "secure" computer somehow, then that fact that it's not directly connected to the public network is meaningless.
The software can jump the air gap as long as there are speakers and microphones on both infected computers.
