Has anyone written an app to transfer data through the headphone jack for P2P data sharing?

Along the lines of "I don't have network and I want to share with someone else on a different platform and don't (or can't) use bluetooth/wifi".

Narrow population, but it would be a fun project to learn about sending data over an audio medium.

Get it working well and you might even be able to send data over a set of headphones/mic, although your data throughput would likely be slow as molasses.