As a guess, the CEC connection being managed by your TV is losing track of the ‘sink’ device that is the Sonos speaker. I’d be doing a few things. First, check the TV and the Sonos system for any software updates, and apply them. Next, reboot both devices by unplugging them from power for two minutes. Plug the TV back in first, followed by the Sonos. Then check the HDMI cable running between them, by unplugging it from both devices, then re-seating it.