Audio Modem Communication Library
This program can transmit a file between 2 computers, using a simple headset, allowing true air-gapped communication (via a speaker and a microphone), or an audio cable (for higher transmission speed).
The sender modulates the input data into an audio signal, which is played to the sound card.
The receiver records the audio, and demodulates it back to the original data.
The process requires a single manual calibration step: the transmitter has to find the optimal output volume for its sound card, which will not saturate the receiving microphone and provide good enough Signal-to-Noise ratio for the demodulation to succeed.