WLAN speaker to speaker pairing
Have you considered using openwrt and 802.11s?  That's the mesh networking built on top of regular old WiFi.  It works pretty well and I've deployed large distributed systems using it.  You can pick up openwrt modules pretty cheaply too.  All you have to do is configure the network and each client then they'll join the mesh and can all talk to one another. That might do what you want.  

The tricky part with systems like that is synchronizing the audio.  You don't want one playing a second behind the one in the next room unless they're far enough apart that you can't hear it.  You could work that problem out though I'd think.

As others said there are systems available today commercially that do what you want but if you want to build your own check out 802.11s.  As a bonus each speaker could become a WiFi access point and hop back to your router.  Of course there's latency and bandwidth sharing but still pretty neat I think.