Are there any error messages? Is this an S1 or S2 system? Are the units wireless or wired to the network? Are you using BRIDGE or BOOST?

S1, S2 did not matter, spent 2 weekends trying to get 3 out of 4 Sonos Connect to Output Audio. No chance of success.  Reboot, Downgrade, Upgrade, Remove, Reinstall, Restart WiFi, nothing worked. One month ago, everything was working without problems.

Received 4 Brand new Wiim Pro Plus and they worked right out of the box with easy set up and sound much better, Even integrate with Google Home fast and easy. Sound quality is light years better than Sonos. 

Goodbye Sonos, Goodbye Garbage