Minimodem, a general-purpose software audio FSK modem

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.