To be quite honest, I don't see the point. It's in every way inferior to wifi and bluetooth. Also, it will, at best, work at old style telephone line modem speeds.
but it's far from new, it's just the principle of a modem connection but using speakers instead of a wire between the two.
Anonymous used some sort of mailboxes/fax with tone dial/accoustic modem in to spread informations to countries cut from the internet.
I often thought of alternativ communication ways. Maybe we can share some ideas about it.
This is where the transceiver enters to transmit over long distances, once the App can encode / decode a headphone cable to the transceiver is only necessary, activating the vox transceiver, this transmit when the app starts to play a sound through the earpiece, I've also seen one very simple circuits that can connect to the headphone output to the ptt is active transceiver, in the pictures below an example of how to connect android systems APRS is.We only need text to transmit. So it's fast enough. But we need to transmit it a long distance...