General-purpose software audio FSK modemhttp://www.whence.com/minimodem/
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, TTY/TDD,
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.