The case of the disappearing Sonos system...

thanks for your reply. Here's my responses. 

How are the sonos connected? Wirelessly? Wired connections do anything different? 

-all wirelessly. I don't have any wired connections nearby the speakers

Lots of issues with Sonos devices and routers. Besure they aren't broadcasting there own wifi as well...

-not sure what that means, do you mean that sonos speakers create their own wifi network? Don't see an option in their app for that..

What is the size of your home? Sq Ft?

-2,880 modern build, kills wifi dead. Hence the satellite & the plug-in satellite. been working just fine for a long time...

What is the distance between the router and satellite(s)? 30 feet is recommended in between them to begin with depending upon building materials.

-router is in the wiring closet, satellite is upstairs and does show valid connection. plug-in is downstairs within 30 feet i'd say

What channels are you using? Auto? Try setting manual channel 1, 6 or 11 on 2.4Ghz and any unused channel on 5Ghz.

-channel 3 for 2.4 (which I believe sonos uses). 
Any Wifi Neighbors near by? If so, how many?

-showing around 10-12...
What WPA security modes are you using? Try WPA2 and AES only and not Auto.

-WPA2 is set

Try disabling the following and see:
MIMO, Daisy Chain, Fast Roaming, IPv6 and Set 20/40Mhz Coexistence to 40Mhz only. Save settings and reboot the router and satellite(s).

If you updated to recent FW v.210, try enabling Daisy Chain. Some have mentioned that this seems to be working in reverse order, enabling means disabled actually.

- that's a lot of settings to mess with, I'll take a look at those :)