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/eARC 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.

Tried the above advice and if you turn of CEC it turns of the Sonos Soundbar. There is only the option for CEC on or off..

Just watched a Dolby Atmos Movie and I had no issues at all. Going to try and standard Sky Channel again and see how that goes again.

Try setting the HDMI-eARC Mode setting on the TV to OFF. Also set Digital Output Audio Format to Pass-Through, not Auto.

Sorry for this again. So turn off HDMI eARC on TV and makes TV Pass Through. Leave sky setting as they are?
 

Set the TV to these settings:

HDMI-eARC Mode - OFF

Digital Output Audio Format - Pass-Through

Keep the Sky settings the same.

Don’t turn off CEC in the TV. Turn it off in peripherals other than the Beam, such as the Sky box (or use a CEC-less adapter). You may however find that the TV no longer switches to the Sky box HDMI automatically.

The Beam has to tell the TV to enter "system audio" mode by sending a CEC command. This tells the TV to mute its speakers and redirect audio and the volume commands from its remote. If the TV loses this setting the Beam won’t play.

Thank for the advice. Turned e-ARC off on the TV and the audio drop outs have stopped completely. The only issue now is that to watch Dolby Atmos Content (Strangly) I have to come out of Sky and use the TV apps installed on TV. The when using Netflix , Amazon, Disney and Apple TV I get full Dolby Atmos.

when I use Sky and its apps then I don’t get Atmos but I sometimes get Dolby 5.1.

i will see how things go. Thank you everyone for your help and kind assistance.

One last thing. I am running the original Sonos Cable (Unmarked). Is there any point in me swapping the cable to a HDMI 2.1??

No there’s not. You’re transporting sound, which does not need high bandwiths.

I’m not sure why eARC needed to be turned off.