I was just checking out Synology's DS audio app & was able to access songs on my NAS. I then noticed that it listed all the players connected to the network, including Apple TV & my Sonos components!
I can now play songs off my NAS on my Sonos system!
It's not perfect, & I can't control multiple zones. The app is also a bit quirky & not as easy to navigate as the official Sonos one. But it works fine until a dedicated Sonos app for Windows Phone 8 is launched.