Since you recently changed from wireless to wired, I’d be thinking about a potential duplicate IP address issue. Power off all Sonos speakers, and reboot your router. Give the router a couple of minutes to come back up, then power back up your Sonos devices.

However, if this doesn’t change anything, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. But don’t post the resulting diagnostic number here, they get sensitive about GDPR.

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 Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Thanks for the feedback.

I wrote my step 3 changes incorrectly.  At step 3, I switched the troublesome Play1 away from using my home’s wifi to being connected by cat6/wired .  That {eventually!} switches the other Sonos speakers to Sonosnet.

Anyway, when I did my testing after step 3 (wireless to wired) I did not wait long enough for the system to settle down.

The other speakers hadn’t switched away from the home wifi to the Sonosnet.

When I returned a couple of hours later, all the Sonos speakers powered up had switched to Sonosnet and now the Wired Play1 can be grouped with the others.

Partially a positive result.

I don’t know why I had to switch a previously working system away from my home wifi.

It certainly takes an undefined (for me) amount of time to switch from WiFi to SonosNet, but then it’s not something I frequently do, unless I’ve run into an issue, which is extremely rare. And of course the Sonos speakers that don’t have the capability to ‘see’ SonosNet don’t  switch.

I have, on a couple of occasions over the last two decades, needed to ‘nudge’ a device to switch, by power cycling it. I assume there was some power surge which made enough difference to the OS to block that process, but a quick reboot always helped. 

I suspect if you move them back to WiFi from SonosNet now, it will work. Part of that process I suggested forces a reload of the Sonos OS on each device, amongst other things, and never hurts.