If you look through all of the previous threads about this, the consensus is that it’s a challenge, due to the nature of the connection of the speakers isn’t a simple switch from one place to another. Surrounds have a parent/child relationship, but stereo pairs are set up as “peers”. Switching between those two states might be extraordinarily challenging.

On the other hand, you can also go in to the surround settings, and set the surrounds from “ambient” to “full” and get them to play as regular stereo speakers, along with the home theater speaker. It’s not quite the same thing as your request, but it’s a passable and already existing function. 

I think the request is stemming from the fact that Line-In on the 300s only works if the speakers are NOT a part of the surround system.

So, for example, my original plan was to pick up two Era 300s and use those are rear surrounds with my Arc. Then attach my turntable which is also on my back wall to one of the 300s to incorporate that into the system.

BUT from my understanding, that actually won't work because Line-In is disabled when the Era’s are used as surrounds. SO for it to work, you’ll need to remove them as surrounds. Which, is fine, but then adding them BACK in surrounds is an effort. 

Which is why, some sort of toggle would be nice. 

(Which, as you point out, I'm sure is no simple feat, but it does limit the versatility of these things just a bit.) 

I appreciate the replies.  I am coming from using a pair of HomePods which did both TV and music very well except for dialogue which is what lead me to get an Arc (and then sub3 and then 300’s).  Part of my rationale for getting the 300’s was using the system for both music and for TV.  As @BRPeterson mentioned, I was hoping to connect a turntable to one 300 to be part of that system.  I agree @Airgetlam that listening to music on the Arc/300 system is not bad but I don’t like the vocals coming from the Arc.  Also, I’m not getting that satisfying bass from the Sub3 now that the 300’s are connected.  I can respect if this is too much of an engineering challenge for Sonos but they have added many great features in their app so I thought it would be worth asking for this feature.  Overall, I’m very happy with the system but if I really want to listen to music, I’ve been going to my den where my pair of HomePods are and listening there.  

I agree with these comments and requests. My surround play1s do a fantastic job when playing music in full mode, but occasionally an instrument or passage comes from the Beam, which is a shame.

A pair of Trueplay settings, for surround and stereo, with a toggle, would be ideal. I know we will never know if it’s going to happen - until it does, if it does - but we can hope!

I think it is a critical feature, it’s very upsetting to say the least that era 300 front tweeters not working when listening to music, and removing them as surrounds every time I want to listen to music is not an option

Well I hoped I that I was just missing an obvious setting, but alas the useless Sonos app is now trumped by this frankly ridiculous setup to need to faff around with taking the speakers out of surround and pair as stereo every time?!  

As a paying customer, I don’t care what the challenge is, it should be clearly stated that this is borderline unusable, Arc and probably the era 300’s going back tomorrow

I just added a Sonos Port for the Turntable and that plays to my Arc Home Theatre Setup and every other Sonos speaker around my Home. I also added a 1Mii B03 Pro ‘long range’ Bluetooth transmitter to the line-out to play TV, or Turntable, audio to my old Sony Headphones… it’s a long range transmitter so can even get a signal out to the headphones, or to Bluetooth speakers, in the rear garden of the Home (and upstairs in the bedrooms too).

So there are actually some good reasons to perhaps add a Sonos Port to a setup. No more unbonding/bonding surrounds here.

Sonos should forget about releasing headphones and instead focus their software team on this problem they still haven’t fixed. 

Wonder which one would generate more revenue. 

Totally.