With the HT room in focus and showing on the ‘now playing’ screen, Goto the "music note’ icon (browse tab) on the main bottom bar in the App and scroll past the music services you have installed and click/select the "TV icon" to restore the TV audio after you have finished playing your music audio… or you can ask a voice assistant, like Alexa, to switch it back: “Alexa, switch mode to TV on Lounge” where ‘Lounge’ is the name of your Sonos HT ‘room’.

Constantly having issues with no sound coming from my arc through the TV. When I finally get sound from local TV channels, then switch to an app like Netflix, Disney etc…. The sound stops working and doesn’t come back. 
 

I feel like I’m constantly having issues similar to this. I’ve had to call customer support multiple times for my system not working. They g to find out why it’s not working is not user friendly at all. Makes me wish I went another route vs this system that constantly seems to not work for me. And the only way to get it up and running is having to be connected to the internet. 

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 Sonos HT device. 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 audio 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.

Most of my earc issues were resolved by switching my hdmi cable to directional hdmi from Audioquest. My main issue now is appletv doesn’t always show up with atmos or even 5.1. Everything coming direct from the Samsung apps is coming through as it should.