Where to start? Can the teensy/arduino record audio? Are there any pre made libraries for this? Can I configure the bit rate of the recording?
That said, I have seen several other crude data-moving tricks from locked-down machines - one faked a USB keyboard and encoded the data in USB HID control packets (used for stuff like toggling the Caps Lock, repeat rate / etc.). Heck, one used a plain non-USB keyboard and literally toggled the data out on the CapsLock/NumLock/Insert LEDs.
If you can fit even the 26 letters in common English usage (not counting numbers or punctuation) into the 16 DTMF symbols, recommend contacting a patent attorney immediately to apply for protection of your intellectual miracle.
For someone who just realized that you can encode data into an audio waveform I don't think it's a bad idea to look academically at DTMF. Being able to abstract both the symbols and the data to bits doesn't seem like a bad thing, and DTMF is a nice approachable example. (Don't want anyone to get stuck on the "how do I transfer my Int over serial, it can only send 1 bit at a time!!!" question