This situation is not unique to the Era speakers, it has always been the case with all surround speakers in the Sonos environment. A similar all of the other threads with this same concern, the best option is to go in to the controllers settings for the Arc’s room, and set the surround speakers to play in ‘full’ mode, rather than ‘ambient’, so they function as normal stereo speakers, rather than merely support for the Arc.

Many of us have found the switching of speakers from normal ‘music’ duty to surround duty to be somewhat onerous, and even more so as you lose any TruePlay tuning you may have done in either room. It can be done, it just ends up being tedious, most likely due to the substantial changes in the nature of connections between a surround (child) speaker and a normal (peer) speaker’s connection.

My personal solution was to buy a second pair that now flank my Arc for music only purposes. The surrounds are surrounds only.

I preferred setting up Era 300 stereos and grouped them the with the Arc. This is not the optimal solution but I assume provides a better end result than setting them up as “full” modded surrounds as proposed. But I expect Sonos to provide a software update to set up Stereo+Center speaker setup for those who prefers a simple - no rear surround speaker setup

Done this way, there is a delay from the speakers grouped with the Arc which results in a kind of echo effect. It must also muddy the sound, as the Eras play 2.0 stereo as the Arc plays 3.0 or Atmos sound. I’m surprised you find that acceptable. 

I did not notice an echo nor a lag but redundant uses of drives which I would prefer a logic to share the roles between center and the stereos. But a native solution is needed for such setup.

There’s no “center” in Sonos, other than the middle speakers in a Sonos soundbar. Sonos does seem to find extra speakers to the side of a soundbar superfluous. I tend to agree with them.

I currently have the latest Arc and SL surrounds.Can I add the ERA surrounds ? Would it benefit my sound?

Add? As in have both Era 300 and One SL as surrounds?  No.  You can swap out the Era 300s for the One SLs, which will give a definite benefit for Atmos, but for regular 5.1, the reviews are mixed. 

So the ERA300's would be a definite upgrade on the SL's?

As I said, for Atmos sources the reviews say they are an improvement.  For regular 5.1 and music, the reviews are mixed, leaning towards they are worse due to the disabled forward facing driver. 

I think I'll leave it the way it is , thank you

I am having the same problem.  For the price (Arc/Sub3/2 300’s), I am disappointed that one cannot have a quality home theater and a music experience.  As @Airgetlam said, removing the 300’s from the Arc and then grouping them into a stereo pair to play music and then having to ungroup the 300 pair and re-add them to your Arc becomes onerous.  While listening to music with the Arc/Sub3/300’s setup does not sound bad, having the vocals coming from the Arc ruins the experience for me. 

Hopefully, the Feature Request to be able to engage the front tweeter on the 300’s for music listening would help but I’m not convinced it would solve the problem.  

My wife keeps asking why I replaced the stereo pair of homepods which did both cinema and music duties seamlessly (for much less money).  If I add more speakers to the room, I think my wife would ask for a divorce.  

How about placing 2 Era300 on each side of the Arc but pretend they are in another room? Thus, you could still use Trueplay for both configurations independently from each other

Sonos feels you must belong to one of two camps: 1: Home Theater, 2. Audiophile. While there are work-arounds, after $4K+ in Sonos speakers I find it beyond limiting; insulting. So if I want bass with my audio, Era300s, I have to remove it from my theater.  Why? Why can’t I add Era300s to my Arc+Bass+OneSLs for immersive theater sound? For a modular system, you are extremely limited and confined to a one or the other philosophy. I thought the newer Era300s would open that door, but alas, no. The disappointment continues. 

I suggest you sell your Sonos and buy something else.  Unfortunately, there are significant differences between the low latency Home Theater configuration needed for video sync and the relatively high buffer delay configuration needed for synchronized audio-only play in multiple rooms.  It’s physics at work, and it’s never going to be easy to switch between the two.  If you find that “insulting”, then I dare say you will not be un-insulted until you move back to a traditional receiver and wired speakers.

The complaints here seem somewhat uncalled for if not inane. There are separate volume levels for music and theater in the 7.1.4 Arc/Sub/ERA 300 setup, and music sounds fantastic when played through this setup for me, particularly so when playing surround/Atmos sources, but even with stereo mixes the combination sounds extraordinary—I just turn up the music volume setting when listening to my setup for this purpose. 
 

Besides that, bookshelf-size speakers such as the ERA 300s are not designed for bass output—that’s what subwoofers are for. Any reputable audio setup has crossovers calibrated to output low end to the speakers designed for that purpose—namely a sub. 
 

I’m not sure what the complaints are about or what you’re looking for—if you spent significant cash building a 7.1.4 setup, that’s exactly what this configuration is giving you. If you were just looking for a stereo system, you likely invested in the wrong system—although I would argue that the Arc/Sub/ERA300 setup is as good as any multichannel receiver is going to offer you, and with the Sonos setup you’re actually future-proofing—because object-based Spatial Audio is where the entire industry is heading. I can’t speak for your feeling “insulted” because I think it sounds fantastic, and you can’t be insulted for paying for a setup that you insufficiently researched. 

Meanwhile, back to the music…

Why is you’re ‘you’ an email link?

People have differing expectations. The best you can do here is hopefully guide them to a better understanding of how Sonos/their system works, but people who have already made up their minds and want to complain won’t be dissuaded. Let those people choose their own path, once you’ve attempted to assist, and they want to argue, it is essentially a lost cause, here on the web.