I operate a bunch of Raspberry Pi's that each run Raspotify and Snapclient. Raspotify is a wrapper for librespot, which enables Spotify Connect to "cast" from

I operate a bunch of Raspberry Pi's that each run Raspotify and Snapclient. Raspotify is a wrapper for librespot, which enables Spotify Connect to "cast" from my (Android) mobile app to each Raspberry Pi (which are connected to speakers of course). The Snapclient instances connect to a central Snapserver instance that also runs librespot (again to enable Spotify Connect). So now I can cast to an individual Raspberry Pi (powered by Raspotify) to play music in a specific room, or I can cast to the central Raspberry Pi that runs Snapserver to get the same music in every connected room. While Spotify Connect is not open, librespot is open source, and it has been working fairly well for me for several years now. One thing to note is that librespot requires Spotify Premium.