MPD has recently gained Snapcast support, Snapcast being the open source Sonos-like multi-room sync system.
I used to run Snapcast but the receiving app has a pretty basic UI (though it has the novelty that one can control the level of any speaker from it).
I have been enjoying the MAFA HTTP stream support. Maybe the addition of being able to receive (and eventually control) Snapcast streams could be an addition for the roadmap?