I would perhaps canvass Amazon, Google & Spotify too to remove any restrictions they have put in place in regard to the streaming services they provide. I suspect Sonos would like this multi-streaming to be available to their own customers, but that their hands are tied to the sharing agreements the providers have put in place. After-all, It means users (like me) may go onto choose to subscribe to more than one Music Service/Account.

Can you explain waht you mean when you say “multi-streaming"?

I think/believe the OP is referring to the fact Spotify only allows the user to stream a single audio stream when making the playback request using voice services like Amazon Alexa for example.

Once a Spotify stream has been started in that way, you cannot then ask Alexa to play another Spotify stream to a completely separate Sonos room, without the first stream being paused - Alexa usually says something like “Spotify is streaming on another device, would you like to stream on Kitchen instead” - just as an example. 

Of course multiple streams are allowed via the Apps, just not when using a voice assistant for initiating playback.

Ah right. Voice commands do seem to access music services directly, circumventing Sonos - and so enabling music services to detect that multiple devices are using one account. I can see this would make it hard for Sonos to resolve, since the whole point is Sonos is circumvented.

This circumventing has advantages though - it makes my Hue lights react to Spotify which they will not do when I use Spotify through the Sonos app.

It should be possible to work this out - using a different approach or reaching out to Google and Amazon - or ask Spotify to have less restrictions in their suppscription.

Sonos use voice control as a selling point and right now the feature is limited, especially when multi room playback is Sonos’ key feature.

I think Spotify would rather have more than less restrictions on their subscription. And this is only a problem when you only work with voice commands - when grouping from the Sonos app Spotify does not notice you have more than one player streaming. This could be seen as unwanted by Spotify though…...

On the other hand; does Google have this feature on on Spotify for  their onw speakers? That would at least mean it is technically possible. 

This is surely a Spotify restriction, and I am almost certain applies to Amazon and Google speaker systems as well. Generally, Spotify (and all the other major streaming services, I believe) allow playback on only one device at a time from a single account, in a reasonable attempt to protect their revenue.

What really surprises me is that using the app it is possible to play several different streams on Sonos using a single Spotify account.  In effect, Spotify treats the whole Sonos system as a single device in this case.

It is completely beyond Sonos’ power to change this situation, and I suspect they would be best advised to let sleeping dogs lie.