I don't usually do this, but here's a particularly interesting Slashdot Comment. This fellow actually played with PSK31 acoustically - that is, just sending it as sound through open air.
Why didn't I think of that?
At least over short distances, it ought to be a lot easier to receive than it is over noisy HF channels.
It's an idea that brings up a lot of interesting questions. How far could it go? How much noise immunity would it have (could you successfully use it during a noisy cocktail party)? Would it do well under water?
It's all fairly impractical, but interesting, nonetheless.