> to have synchronised compressed audio streams play on multiple speakers with do audible delay between them. That is not a partially "easy" problem to solve,

> to have synchronised compressed audio streams play on multiple speakers with do audible delay between them. That is not a partially "easy" problem to solve, Without commenting on the radio side of this, I implemented automatic synchronisation of video playback units to within a few microseconds in 2007 and it was fairly obvious how to do it back then. That's accurate enough for audio as well. So long as units were playing the same content or equivalent-length content (for video walls, shows etc.) and could receive each other's broadcasts on whatever LAN they were connected to, they would automatically detect each other, form equivalence groups, and adjust timing until the clocks matched each other, and stay locked together across content changes. There was no master control unit, and no configuration; it was an automatic distributed algorithm that "just worked".