Hacking Headset jacks and data cables

[OneMichael]

I was looking at an old computer, a little while ago, which actually sends programs out its speaker jack to a tape deck. You can loop that deck around to the microphone jack, and load the programs back into the system.

So I wonder: You could potentially tell your nds program to encode bits into a stream of sounds, like what your modem uses to dial in. You can accept information through the microphone, too.
Assuming you have something set up to capture the incoming data, how fast could the data be transferred?
While I'm at it, how much power is able to be drawn out of the headphone jack?