Shannon-Hartley says the theoretical maximum data rate for a channel with AWGN is proportional to bandwidth and the log of signal-to-noise ratio. For an off-the-shelf microphone/speaker pair, I think 16 kHz and 80 dB are probably decent guesses. That would give a theoretical maximum data rate of about 425 kb/s. The practical limit is probably much lower.

It may be possible to increase the bandwidth by increasing the sample rate on both ends, but this quickly leaves the realm of consumer audio equipment (and consumer pricing). At some point you'd exceed the reasonable frequency responses for each device, as well as the medium. I imagine that air attenuates ultrasonic frequencies more than lower ones, but that's just a guess.