By the way the older address https://labs.rakettitiede.com/?p=87 reported security warning:
“Varoitus: mahdollinen tietoturvariski
Firefox havaitsi mahdollisen turvallisuusuhan, eikä jatkanut sivustolle labs.rakettitiede.com”

http://firebay.pelaaja.info/embedded/20192205/12kbps-simple-audio-data-transfer-for-avr/

We’ve all been there: after assessing a problem and thinking about a solution, we immediately rush to pursue the first that comes to mind, only to later find that there was a vastly simpler alternative. Thankfully, developing an obscure solution, though sometimes frustrating at the time, does tend to make a good Hackaday post. This time it was [David Wehr] and AudioSerial: a simple way of outputting raw serial data over the audio port of an Android phone. Though [David] could have easily used USB OTG for this project, many microcontrollers don’t have the USB-to-TTL capabilities of his Arduino – so this wasn’t entirely in vain.

Serial communication via audio on Android
https://davidawehr.com/blog/audioserial/
At first, it seemed like a simple task: any respectable phone’s DAC should have a sample rate of at least 44.1kHz. [David] used Oboe, a high performance C++ library for Android audio apps, to create the required waveform. The 8-bit data chunks he sent can only make up 256 unique messages, so he pre-generated them.

Oboe is a C++ library which makes it easy to build high-performance audio apps on Android.
https://github.com/google/oboe

Serial Data from the Web to an Arduino
http://hackaday.com/2015/10/10/serial-data-from-the-web-to-an-arduino/

In the old days, a serial port often connected to an acoustic coupler that gripped a phone handset and allowed a remote connection to a far away serial port (via another phone and acoustic coupler) at a blistering 300 baud or less.

In a decidedly retro move, [James Halliday] and [jerky] put a modern spin on that old idea. They used the webaudio API to send serial data to a remote Arduino. The hack uses a FET, a capacitor, and a few resistors. They didn’t quite build a real modem with the audio. Instead, they basically spoof the audio port into sending serial data and recover it with the external circuitry. They also only implement serial sending (so the Arduino receives) so far, although they mention the next step would be to build the other side of the connection.

serial over webaudio
http://substack.net/serial_over_webaudio