This is a post to try and help people with their Sonos system dropping off the WiFi network on a regular basis when everything else on the network seems ok. 
 

I contacted Sonos and the technician solved my long term issue. It required me switching off the guest WiFi on my router which I did by logging in as administrator. Easy as that! All now works fine. Apparently my guest WiFi was using up too much bandwidth. Hope this might help.