Since that’s the tv menu, what does Panasonic suggest to fix it?

Does it reappear if you temporarily disconnect the optical cable?

Is this recent behaviour, or was it always like this? Is it an older TV? What source are you playing from. Is this source connected through HDMI? Older TV’s (especially Panasonics, I notice) often do not send surround from HDMI connected sources to the optical connection.

It’s a Panasonic Viera plasma from 2012, model TC-P55GT50.

My previous soundbar was an Energy one. When I had Optical out from the TV to the soundbar, the TV’s Surround menu was active, and I could set it to 2.0 or 5.1 (despite the soundbar not having surround qualities).  

Using the Sonos. optical to HDMI adaptor, the TV’s Surround menu is greyed out - it does not see the Sonos as a 5.1 device, only 2.0.

I have a case open with Sonos, as they assured me before my purchase that I could get 5.1 with that TV model using the optical to HDMI adaptor, and clarified that if I used HDMI only, I could only get 2.0.

Same with the Blu-Ray player, same with anything from Firestick.

Had tech support on the phone last night for over an hour, they had nothing. Hopefully I’ll hear back today, I’m not very happy at the moment...

Sonos had told me (before I purchased). - 
Your Panasonic Viera TV TC-P55GT50-2 Plasma TV from 2012]can work with the Sonos Arc Ultra using an optical audio adapter. However, using the optical connection means you won't be able to control your TV with CEC or voice commands.
For 5.1 surround sound from your Amazon Firestick and Blu-ray player, ensure they are set to output Dolby Digital 5.1. The TV's optical output should be able to pass this through to the Arc Ultra. Keep in mind that Dolby Atmos requires HDMI ARC or eARC, which your TV doesn't support with optical

Yet I spend over an hour with Sonos support last night to no avail. They were supposed to escalate but did not. I called again, and they said “you’ll get an email in 2 business days.”

Someone on Sonos forum contradicts what Sonos told me -

Seems I’d need to get an HDMI switch and the HD Fury Arcana to solve my problem. I’m in this deep, do I have to go in further?

VERY VERY unhappy with Sonos right now.  

I found repeated information that the only way to solve this is to get an Arcana HDFury so I can bypass the TV and get full Atmos. Box has a single HDMI input and two HDMI outputs -one of which is full eARC and goes to the Sonos, and the other goes to the TV for video. If it works, I’ll probably add a switch for the input, but I mostly use the Firestick anyway, hardly ever the Blu-ray, so I can just switch the connector when needed.

Something else has changed, either a TV setting or the devices you are now connecting to that TV.

Older TVs like this often cannot pass DD 5.1 over optical from external devices, but can pass from internal sources (eg TV tuners).

There is literally nothing Sonos can do about this: there’s no “capability detection” over optical, they play what they are sent, and so some reason your TV is sending stereo. Its Panasonic you should be calling/yelling at, though for a 12 yr old TV good luck with that.

I understand that Sonos cannot do anything about this, but they told me it would work with my model, so I’m still a bit pissed off.

I bought an Arcana HDFury, which came highly recommended from other users of Sonos.  It’s not quite working yet, getting support from the Arcana people.  The HDMI input to Sonos ARC is now a full eARC connection, but something’s not “clocking” correctly - I’m only getting a split second of audio once in a while.

I still haven’t heard from Sonos, I escalated this last Friday, 12/20.

So, right as the holidays, and some people’s time off started occurring.