How can I use Downcast with my Sonos system?
Currently, the recommended way is to connect your Sonos system to an Airport Express device on your WIFI network. This will allow you to play audio through your Sonos system via Airplay. It's unfortunate that an Airport Express is required but including direct support for Airplay in a non-Apple device such as Sonos would require a direct agreement with Apple.
Instructions for making Sonos available via Airplay: https://sonos.custhelp.com/app/answers/detail/a_id/1241
If I can access Spotify, Pandora, etc through Sonos why can't I access Downcast?
These apps are all web-based services, which is what Sonos supports when it comes to 3rd party systems. Downcast is not web-based, it is a stand alone app. Your subscriptions, settings, and downloaded content are stored directly on your iOS device or Mac so there is simply no service for your Sonos system to connect.
Will Downcast ever work on Sonos?
That's the plan! The next major version of Downcast will have some significant changes, one of those being server-side functionality that would make it possible to add support for Sonos. Stay tuned!