I was reading the story "Despoilers Of The Golden Empire" by Randall Garrett when the author mentioned that the titular empire could transmit news quickly over long distances without horses. This made me list the fast communication methods available for a pre-industrial civilization in this context and lead me to an acoustic equivalent of the optical telegraph, with its corresponding strengths and weaknesses.
I based the modern version of the system on existing technologies used for acoustic radar and mechanical audio amplification.