alt Hacker NewsYou can do PSK31 across a room with just audio, but it is a kind of annoying noise after a while.
It's interesting that you learn to recognise patterns in the burbling tones after a while. "CQ CQ CQ" is very distinctive and after a while you can hear your own callsign in it. I guess it's not that different to very fast Morse.
Or SSTV! I recall a demo years ago where someone had SSTV running between a speaker at the focus of a small dish, across to the room to another small dish with a microphone...