Minimodem is a command-line program which decodes (or generates)
audio modem tones at any specified baud rate, using various framing
protocols. It acts a general-purpose software FSK modem, and includes
support for various standard FSK protocols such as Bell103, Bell202,
RTTY, NOAA SAME, and Caller-ID.

Minimodem can play and capture audio modem tones in real-time via the
system audio device, or in batched mode via audio files.

Minimodem can be used to transfer data between nearby computers using
an audio cable (or just via sound waves), or between remote computers
using radio, telephone, or another audio communications medium.