What you can do though is essentially create a modem that would encode digital data into an audio stream, and through that send encrypted audio packets. If we assume perfect audio quality, a software implementation of a 56k modem would give you roughly 50kbps to play with, over which you can send the encrypted audio from a low-bandwidth audio codec like those used by VoIP.
I am not aware of any out-of-the-box solutions for this, so you'll have to build most of it manually. To be honest, getting the audio in and out is the easy part; doing the encryption and modulation/demodulation is the hard part.