If you'd like to play radio stations and the many Spotify playlists, here's what you can do:
1) Download the following 2 ZIP files:
You're basically turning your PC into a streaming music server.
"Dsbridge redirects the DirectSound API and exposes the playing stream as an MP3 stream with metadata"
2) Find the location of your Spotify.exe file on your drive. For Windows 7 users, it'll probably be in C:\Users\
3) Copy dsound.dll, dsbridge.ini, and lame_enc.dll from the ZIP files into the same folder as Spotify.exe.
4) Find the IP address of your PC. You can do this from the commandline by entering ipconfig.
5) In Sonos, create a radio station with the following parameters:
6) Start Spotify on your PC. Turn down the volume in the Spotify app.
7) Start the Spotify radio station in Sonos
As it is a radio station, you have no control to FF or change music genres in the Sonos app. You'll have to do this on your PC from the Spotify application.
You should create a DHCP reservation for your PC so you don't need to change the IP address in the radio station settings whenever you reboot your computer.
Other than that, it's pretty stable, as long as you have a reasonably clean wifi network. I believe this will also work with other music services, although I haven't tested it. You'd have to modify the Dsbridge.ini file to reflect the name of the executable that you're interested in wrapping.