It initially sounds like there maybe something else connected to the TV which is attempting to steal away the CEC focus from the HDMI-ARC connection to the Amp. If you have any ‘other’ devices connected to the TV HDMI ports, such as Apple TV, Fire TV, Cable TV or a PlayStation etc. (...to name but a few of many such devices), then I would first try switching off CEC control on each of those peripherals (leave CEC enabled on the TV obviously) and then see if that resolves the issue.

Note you should ‘ideally’ power OFF the entire TV/HT setup, after disabling CEC, as control using the TV’s remote can sometimes persist after changing the setting on some devices.

If it solves the issue, then you could then go onto try re-enabling CEC on one device at a time and see if the problem returns, then just leave CEC switched OFF on that one device only.

Note some connected devices may occasionally not provide any settings to easily disable CEC, however this can still be addressed by using a CEC-Less adapter between the device and the TV. See this link for such an adapter (shown here, just as an example - other adapters are available too):

https://www.amazon.com/Lindy-HDMI-Adapter-Female-41232/dp/B00DL48KVI

Anyhow I hope the above suggestion helps you to resolve the matter. If problems persist, then I would recommend when you next encounter the dropout issue, to submit a system diagnostic and note, or post, it’s reference back here and then contact/chat with the Sonos Support Staff via this LINK and see if they are able to suggest some further things to perhaps resolve your matter.