What about something like this?
I mean - that's just the first hit I found searching for open-source multi-room audio streaming for the raspberry pi. There were others.
Seriously - once you got one node to work, at that point clone the sd card (just dd it), and pop the image onto new cards for each new node. I don't know what kind of configuration or fiddling you might have to do, and while it isn't a "plug-n-play" thing like Sonos, you (and everyone else here) probably have the smarts to figure it out.
And once you have it figured out for one - the copying it and duping it to the other nodes and setting them up is probably not much of a stretch.