FM radio from your computer. It is kind of built-in to the rasppi GPIO clock output. proper antenna design.
Networking over audio, or even low-bandwidth radio is a whole fascinating enthusiast community on the internet. Airchat is a low-bandwidth acoustic chat wossname targetting FM type broadcasts. Initial reviews were negative. fldigi is possibly the leading generic data transfer tool here. Typical modes not fast; you might be able to type faster. soundmodel is another suite of audio transmission tools; the maintainer has vanished. an example of usage shows what can happen here. Packet radio is built on soundmodem.