I really need an open alternative -- sonos speakers run linux under the hood (AFAIK), and I would be surprised if there is not a RPi powered project somewhere. The speaker hardware is perhaps the hard part.
I agree there should be a compute platform somewhere to do this, but having listened to (and owning one) a few Sonos products, I think the technology is the less interesting part of the deal. Their speakers sound really good, and seem to make my sources sound really good in a really easy to use package. I have their AirPlay 2 compatible soundbar, and it works so well when hooked up to my TV as the input, and seamlessly can then receive AirPlay from my devices and fill a room. If there is an open source equivalent, or plans to make one, I would love to see it paired with really amazing sound producing hardware.
It's a bit annoying that Logitech killed the squeezebox, which made it somewhat more accessible, but I believe that thanks to that the code is open source now, so there's that.
I run Snapcast across my house connected to MPD running on a local Linux box, and have been pretty happy with it. I've had to do very little maintenance to keep things running.
The main thing I haven't been able to figure out is how to have multiple Snapcast streams and control which room listens to what. I don't actually think it's possible to do.. (though I could be wrong?)
Input streams are configured once on the server. You can group clients together and assign a stream to a group.
This can be done either with Snapdroid https://github.com/badaix/snapdroid or with Snapweb (is shipped with the Snapcast server since version 0.21) https://github.com/badaix/snapweb or with any other control client from the community
