[snthd]

https://github.com/badaix/snapcast/blob/master/README.md

Not a solution, but snapcast is a useful part of the puzzle for the actual streaming and multiroom sync.

[voltagex_]

What's the latency like? I've got some Qualcomm Allcast gear that seems to do sync properly with no perceptible delay, but it's the only one I've heard that does it properly.