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.
It’s pretty old skool now but it works just fine. Has plugins for airplay and whatever android does as well.
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?)