If the source is 2.0, then there isn’t anything that’s being sent to the surrounds, as there’s no surround “channel” involved. 

It’s unclear from your post about 5.1. What’s the source you’re playing? I’ve had various local “shows” that are broadcast with a 5.1 signal, but only carry  a stereo source, so my surround speakers are receiving “data” of no input. Your test should be, if it isn’t currently, some recent action movie that carries a 5.1 signal. Things like local sports, talking heads,  or news programs aren’t always sending that surround signal.

The implication is that you’ve tried several sources to test the non-Atmos aspect of 5.1, that’s good, bearing in mind my previous statements, but have you submitted a system diagnostic within 10 minutes of experiencing this problem, and called Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network. 

Thanks for this. No I haven’t submitted a ticket with Sonos - thought I’d try here first.
I’ve tried a few different sources on Netflix, Amazon, Disney etc where 5:1 is stated as an output. When streaming I’ve checked that the audio stream is set to 5:1 and opened the Sonos app to confirm that the Arc is receiving 5:1. All this is set - and I figured that if the Arc knows it’s getting 5:1 it should be utilising the Era’s at the rear but they remain silent.

On the 2.0 subject - this was a long-shot but wasn’t sure if there was some way to have the 2.0 output play emit from both the Arc and the Era 300s. I think the only way to do this is by grouping them instead of using the Era’s as surround - but doing so wouldn’t get the most out of the plethora of media that uses 5:1, 7:1 or Atmos.

So, I agree with your first paragraph, now that you’ve added some extra data to fully understand And this is why I’d probably be talking to Sonos, too, rather than just this community. As helpful as many folks here can be, the fact that it works with Atmos and not with 5.1 is somewhat concerning, and would likely require some reading of that hard data in the diagnostic that none of us users can see. 

And yes, I also agree with your second paragraph. Sonos just doesn’t do any upscaling of signals...it relies on other devices to feed it what it needs. If you could get your TV or whatever is feeding the TV to do that upscaling, the Sonos would be happy to play it. I certainly “get” the desire, though :)

Figured I’d give an update here in case others have similar experiences. I’ve just been talking with Sonos (full disclosure, I’m employed by Sonos but in the IT business operations and waaayyy removed from the magic in the products).
So, in the app, in the ‘Surround Audio’ subsection, I cranked the ‘TV Level’ output to the max. I can then hear output from my rear Era 300s when playing 5:1 audio. It’s still pretty quiet.
Using Trueplay was suggested, but I’m an Android device man and don’t have any Apple products so can’t use this until the feature comes out for us Google Store users.

Finally, I think my issue would be solved if the veeerrryyy popular feature request is implemented; to enable use of the Era’s central speaker in surround audio. Fingers crossed this is worked on and not just for music playback 

Plus plus. I'm going nuts with the uselessness of 5.1. MY PS5 somehow only outputs 5.1 to it instead of 7.1

I should probably update;  we got the Sonos app installed on my wife's work phone which happens to be an Apple device. 

We did the Trueplay calibration using this and it somehow made all the difference. Have had no issues since 

I have been fighting this for hours after getting in the era300 today. Disappointed 

I've also just started having issues, not sure if there was a stealth update? Had eras since launch and work well for Atmos music and movies. The last couple of days, they keep having green lights to suggest being muted? I've turned them off and back on and the light changes but I'm not getting any sound out of the via cable tv (sky UK), LG netflix app, amazon music or 4k blu... I tried to remove and re-add but they won't remove, guess I'll do a hard reset and reconnect. 

What does the app say you are getting, soundwise?

It reports the appropriate codec/format but nothing is coming out of the rears. I've factory reset and added them again and sound comes out during setup. Once added though, can't hear anything with spatial music, a 4K blu or the Series X sound test... 

I'd make a diagnostic and call Sonos.

By the way, I do not think Sonos does “stealth” updates…...