This sort of stuff sounds a lot like covert channels, like how firmware hackers program a bootloader that blinks the power LED to obtain the original firmwares from devices.
Some interesting reading: http://www.flakelabs.com/index.php/blog/
They're using an Android device to connect to a computer via audio, they use a circuit which is relatively complicated compared to mine. My implementation uses a few resistors and a capacitor then uses the hardware built into the AVR to do the rest of the work.
Other hardware related methods include:
- adding a microphone on the Teensy to read audio played by the computer's speakers (unreliable, complicated circuit)
- adding a LDR (light dependent resistor) on the Teensy and sticking it on the monitor or an LED on the computer (simple circuit but slow transfer speed and fiddly)
- adding a bluetooth module to the Teensy (expensive, requires host computer to have bluetooth)
- adding a WiFi/Ethernet module to the Teensy (expensive, plus many other complications)
I'd say direct connection to audio out is the best because it's readily available and the circuit to interface to the Teensy is very simple (the way I'm doing it). So to actually answer the question, yes it is possible to use audio to transmit from the computer to the Teensy, as I said in my original post the device can be any that can play or generate audio.
I've been meaning to do more work on this project but I've been busy with uni assignments, I have a little bit of spare time this weekend so hopefully I can at least release something for public use before things get busy again.