Only took me 7 months (plus one productive morning!) to fix. Turned out not to be a Sonos problem, but will leave here under Hardware > Sonos just in case others find themselves similarly afflicted.
I believe to have posted this in at least one other forum before as it is not new data but for those who use SONOS, I would recommend to avoid using SONOSnet and move them to wires and wifi as much as possible.
See the spectrum results below of my SONOS system on channel 1 when it is playing captured while sitting on my couch in the living room:
And this below is from the same location while it is idle. You can see how chatty SONOS is as it completely overwhelms its channel.
Of interest, the last spike to the right was from my now defunct Plantlink Zigbee Channel 26 network. Many US made Zigbee devices do not support this channel as I found out the hard way. It presents the advantage of not interfering with any of the US WIFI channels though which stops at channel 11.
The sharp spike in the middle of the spectrum was from my Philips Hue network which was on Channel 15. Zigbee channels are much narrower (5MHz) than WIFI (40 or 80MHz) and do not overlap like WIFI does.
If I turn on the microwave the whole spectrum is blanketed with noise...
Eliminating SONOSnet eliminated the entire channel 1 spectrum without significantly adding load to the wifi one which you can see here other broader channel on WIFI channel 11.