What other troubleshooting steps have you gone through so far? Has the Arc/Eras already been factory reset for example? Have you tried a different router (if you have another available) or have you already recently switched or perhaps factory reset your router?

I ask those few things, because when ‘bonded’ the Era surrounds each connect over a 5Ghz direct wireless connection to the Arc and their IP addresses are assigned by your router, but proxied via the Arc. A few routers can have issues with the proxying of the IP addresses and sometimes a router factory reset can occasionally resolve the matter, but it would be nice to know more about what action has been done so far. 

I’ve factory reset the eras, unplugged and replugged them in at least two dozen times, unplugged and replugged the Arc, reset my router and modem. 
 

The Eras were set up and connected as surrounds, and then mysteriously today became an issue. 

What make/model is the router?

To perhaps save time communicating back & forth. Here’s my first suggestion…

I’m guessing you factory reset the Era speakers before first removing/unbonding them from the Arc? I’m just wondering if the Arc perhaps thinks they’re still bonded as ‘offline’ surrounds? 

It seems you’ve reset everything so far except the master HT device, so maybe consider an Arc factory reset, but do not (ever) reset all your speakers in one go, as you could lose all your system settings - you need to leave one speaker setup and working in the Sonos App so that it retains all your settings.

Once the Sonos Arc is added back to your system, try ‘bonding’ the Era’s again to it. Hope that works for you.

How do you mean by “master HT device”?

have you experienced this before with your system and was this the fix? I do appreciate the assistance but am really looking for someone who has experienced this and/or fixed this issue versus more guessing, after having spent 4 hours doing that already today with Sonos. 

The HT ‘Home Theatre’ device - which is your Sonos Arc controls the secondary devices when they are bonded to it. The secondary devices in your case are the Era’s - your system was setup and working okay and you mention you have already factory reset your modem/router, the two Era devices, but so far you have not factory reset your Sonos Arc, so it makes sense to try that next.

As mentioned when bonded as a HT setup, the Era products switch and communicate with the Arc over an Ad-hoc 5Ghz wireless connection and the router DHCP IP addresses are assigned by proxy, via the Arc, to the Era’s - if your router has been reset then one assumes the DHCP server is working correctly as your Era’s and Arc are working okay when they are not bonded - so my suggestion is to next try the one thing that you have not done already and that is to factory reset the Arc - as that’s the device responsible for the proxying of addresses and the ad-hoc wireless connection. 

Sorry but I thought you were looking for suggestions to save you waiting for Sonos Support? If you don’t want to go down the suggested route then that’s fine and perhaps you are best to wait for level-2 Sonos Support to get in touch with you next Monday. I’m just expressing a view of what troubleshooting I would have tried in the meantime.

I appreciate the help. I will try resetting the arc and see if that works. 
 

the Eras stay working when they’re not set up in surround, but now when I try to connect them as surrounds I get the “not connected” error, so I can see why that might make sense. 

I’ve now factory reset my arc, and still cannot get my era speakers to connect back as surrounds. They still work separately though. 

Ah well it was worth a try - can I just ask what router you are using and do you know if it’s firmware was recently (auto)updated?

Also, I assume your Era-300’s are not stereo paired at the moment.

Correct, they are not 

Assuming the Sonos hardware is okay then the next thing I would try, is a Sonos App reset ‘Settings/App Preferences’. See this support link: https://support.sonos.com/en-us/article/reset-the-sonos-app

On reopening the Sonos App, post reset, accept the T&C’s and connect it back to the ‘existing’ Sonos system.

If that doesn’t solve it, then it’s time to look at the router settings/firmware, but I would need the make/model to do that here and check to see if it’s firmware has been updated recently.

ARC + (2) Era 300s. Set up ARC, added Eras as separate rooms, tested music playing through each three. All three on wifi; Comcast all-in-one modem so theres no picky firewall stopping anything. Added Eras as surrounds and app says it’s all good. Start streaming movies and no surround audio. Check app and it shows the two Era 300s as NOT CONNECTED. I’ve reset and gone thru the setup four times already. I’m assuming there’s a firmware bug causing this or is the Eras as surrounds just not ready for showtime?

All three setup as separated rooms, grouped them under the ARC playing tv audio. Somehow the app forced the Eras to populate as surrounds in the app but they’re still not playing any audio

What audio codec is playing to the room? If it’s PCM 2.0 then you’re not going to get any audio output from the rear channel speakers, you need to provide more detail about what the TV/Peripheral is sending to the Soundbar. As a quick check the ‘Now Playing’ screen in the App will show you the input to the Arc or you can goto ‘Settings/System/About My System’ and find the Arc in the device list and when it’s playing checkout what’s showing next to Audio In:

Ideally, the codec needs to be DD or DTS 5.1 surround sound or similar, not stereo audio.

@Ken_Griffiths i have to get past the Eras showing not connected before moving in to that. Or are you implying that they show as not connected when in stereo?

I don’t quite understand, as you did mention this earlier… ‘Somehow the app forced the Eras to populate as surrounds in the app” ..? 

@Ken_Griffiths yes. So last reset had them all as separate rooms, grouped them together and the eras showed up as surrounds in under the Living Room ARC. Played Guardians of the Galaxy and still no audio out of the Eras. Tried full channel music thru Apple Music and still nothing. I’ll try again later this morning and see where I get

Maybe describe (or screenshot) what you’re seeing as your Sonos Arc Room Name in ‘Settings/System’ in the Sonos App. Also check the ‘Audio In’ on the Arc room, as mentioned earlier.

Ensure you’re ‘bonding’ the surround speakers to the Arc through the Arc ‘room’ settings in ‘Settings/System/iArc Room Name]’ and not simply ‘grouping’ the speakers together in the System/Rooms main tab.

If you’re TV/Source device is not sending out the correct audio, then you may only get stereo by default anyway, so check you’re ‘Sound Out’ settings, particularly on the connected TV itself.

@Ken_Griffiths ill do that

@Ken_Griffiths 

reset app, reset devices, entered each one as a separate room, added Eras as surrounds. Appletv set to Dolby 5.1, tv arc set to pass through. Still no audio from surrounds and it shows Eras as Not Connected. 
 

should I have a dedicated 5ghz wifi for the Sonos?

The Era’s should be connecting to your Sonos Arc ‘direct’ over an ad-hoc 5Ghz wireless connection when setup and bonded to the Arc, did the two successfully setup as the left/right surrounds? - What ‘rooms’ show in the Settings/System area of the App, below ‘Products’ ?

@Ken_Griffiths 

i understand that the Eras run ad-hoc thru the ARC. The Eras are no more than 10ft from the arc with line of site. But should I have a dedicated 5ghz or 2.4ghz for the Sonos? I’ve been seeing a lot of iot devices only running on 2.4ghz recently and I’ve had to separate SSIDs for 2.4 & 5 - I’m a AV custom integrator by trade so I wanted to test the Eras as surrounds before I start suggesting them to clients. 
 

Just to say, looking at that last screenshot, my own early thoughts on this issue, are that your router ‘perhaps’ may not support the Arc proxying the Era-300 IP addresses when they are bonded to the Arc over its 5Ghz Ad-hoc wireless connection. One or two (a very few) routers have been known to have this type of issue. It can often be resolved however by cabling just the Arc direct to the router in some instances - so if that’s practical in your setup, you may want to give that a try at some point. Whatever you do, ensure you do not switch off the Arc’s WiFi adapter when it’s cabled, as the adapter is required to ensue the communication with the wireless ‘bonded’ surrounds. 

If it’s not practical to wire the Arc to the router, then you can also try wiring another Sonos device instead, but not a surround speaker, or Sub - Then wait 10 minutes for all products to switchover  to a SonosNet signal .. devices will show WM:0 in the ‘about’ box in the App, next to their entries in the list when they have switched and are running on SonosNet - If for any reason a device does not switchover, then perhaps power-cycle it and see if that sorts things for you.