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.