Im currently building my home theater sys with Sony Bravia 65 A80L Sonos Arc, Sub Gen 3, SL 100 and just added 2 Era 300. All of my devices are hardwired except for the Era 300 due to the placement, Wall mounted 8 feet from the floor. Once i got everything firing fine, switched to surround then the stuttering (pulsing ) sound coming from the 300s every 5 seconds, otherwise zero sound issues outside of Surround. After several hours of troubleshooting, power cycles, tweaking settings in the app and factory reset, all to no avail so then i started restoring the 300s back into my system but accidently left the ethernet cable attached to the speakers. The setup process recognized the hardwire connection and also gave the option to connect for Wi-Fi. I proceeded with the hardwire connection and that solved the stuttering sound issue. I guess the system does not like having it’s devices split between wireless and hard wire connections. BTW, there might be an issue with the software indicating Line-in Connection when using the USB-C dongle for ethernet connectivity, the function is there but the software says “Not Connected”. Sonos if you listening, please fix this. Add the Disable WI-FI / Enable WI-FI function for the Era 300
Some points to this…
If used as rearspeakers in a home theater system the line in function of every sonos device that has one is disabled. This also for the Era devices using the adapter.
Normally you can ise mixed connections, wired and wireless, if you don’t „break the rules“. 
I suppose when you used the Eras non wired as rearspeakers the wifi of the Arc maybe was disabled, because it is wired. Could this be? If yes, you would have to ebable it because the Arc in this mode tries to build up an own 5 GHz network exclusively for connectivity to wireless sub and rears.
Only in case ALL home theater devices are wired this wifi network isn’t needed.
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