It sounds like a HDMI-CEC issue - perhaps consider switching off CEC on all your ‘other’ HDMI connected devices and just let the ARC use the feature. See if that maybe resolves the issue for.

That suggests the TV is not properly sending a signal out via ARC or eARC to the Sonos to play. I’d first check to see if there were any software updates to the TV itself. Then as a precaution, I’d reseat the HDMI cable at both ends, and finally reboot the TV by literally unplugging it from the wall for three minutes. Just powering it off with the remote or buttons on the device is not enough, it needs to be fully disconnected from electricity. 

Thanks Bruce, I'll try it out!