When I was doing research for the Audiofile articles, one of the surprisingly discoveries I made was the degree of overlap between audio sampling and other kinds of scientific sensors. In retrospect, it's obvious that the theory behind accurately measuring an audio signal thousands of times a second would be much the same as taking a measurement from something like a temperature, voltage, or orientation sensor. But it takes a moment's thought to make the transition from audio-as-sound to audio-as-voltage (or audio-as-binary-stream), which is the paradigm shift that makes it possible. It's not just a microphone or a speaker--it's an I/O port.
Audio inputs and outputs are everywhere, but they've fallen out of style for digital interconnection, which is why it's so cool when someone uses them in unconventional ways. One of my favorite examples is the re-release of Bangai-O for DS, which lets users trade custom levels through audio files (they sound a bit like picking up the phone on an old modem). That's really clever--particularly since it takes nerve to ignore the console's built-in wireless connection (a good thing, given the way Nintendo has reliably squandered it). Sharing MP3 files over the Internet is cheaper, easier, and longer-lived than any centralized, developer-provided solution could have been. It even degrades gracefully (you could send creations by mail via cassette tape).
Another great use of audio hacking showed up on Make recently, with a point-of-sale system that plugs into a smartphone's microphone jack. You see something like this and you think "well, of course!" It reminds me of the old IR blasters that were available for PalmOS back in the day--plug a stubby, square rectangle into the headphone jack, load up some .wav files, and suddenly you've got a universal remote control. Both inventions play on the realization that almost every device has a high-quality digital sensor with a sampling API and a standard pin configuration, as long as you stop thinking of the headphone jack as "for music only."
Tricks like these are not only fun for digital audio nerds like me, they're also a reminder that audio is a big part of our software heritage. After all, the original sytem hackers were audio people: phone phreaks who exploited flaws in the network signaling to get free long-distance calls. It's been a long time since those days, and since you had to physically place a handset onto an acoustic coupler to go online (that was even before my time, actually), but audio remains a powerful (and evocative) tool for storing, transmitting, and even hiding information. I can't wait to see what people will invent (or revive) next.