The amateur radio community is chock full of innovation for low bandwidth weak signal decodable comm protocols.

There's also V.xx modem standards that are kinda dependent on the characteristics of the phone lines, but might work for audio at a distance?

ham optimizes for the wrong thing, imo. look at ft8: perfect for making contacts at low power with stations far, far away, but really only tuned to the particular task of making contacts.

you can package some text alongside, but fundamentally all amateur operators are looking for is a SYN / ACK with callsigns.

There's also JS8call which is a modified version of FT8 meant for actual communication. IIRC you can do some neat things with it, like relaying a message through another user if you don't have a direct path to the recipient.

As one of the accursed hams, I wonder what ggwave's propagation profile would be compared to RTTY / CW (Morse code) etc. Would be interesting to try it out.

RTTY is the sound of "satellites" in a lot of media.