When can we listen to the advanced crossfade feature via Sonos (Spotify's new "Mix" feature)

I’m not sure Sonos offers the process as Spotify uses it in the SMAPI. You might want to check the data here, but Spotify is ultimately responsible for the data they send through SMAPI to your Sonos devices. If there is the option, Spotify would need to implement it on their servers, and pass through the data. Sonos effectively points your system to Spotify’s servers, but wouldn’t have access to change any programming on it. Spotify itself writes the interface you see, using the Sonos SMAPI. 

I appreciate your help Bruce, “Mixed playlists can only play on speakers or external devices via Bluetooth at the moment and since Sonos speakers use WiFi and Spotify Connect, that's why you can't get them to work with this feature.” (Source: Re: Mix Edit and Transitions - The Spotify Community)

Anyone else facing this issue - Please vote here :Update the Mix feature to work on Sonos/Google Nes... - The Spotify Community

It would be nice if Sonos were to institute (again?) a voting process. While adding a voice to the request certainly doesn’t hurt, I suspect ‘public’ forums are rife with the potential for misconduct when ‘voting’.

Yeah, those public voting forums never worked out well for Sonos.  Full of one and done voters from fan site call-to-arms, a few die hard promoters who got toxic with anyone who dared offered a reason why their idea for a “Sonos enabled espresso machine” wasn’t a major priority, and the inevitable closing of the threads and/or forum.  

Instead, Sonos seems to rely on the far more relative, not to mention accurate, private polls that it sends out to known users of the product. 

Would this limitation suggest the mix or transition between streams is actually on the device running the Spotify App playing the stream(s), eg the phone, rather than the Spotify servers?