What comes out of the back channels depends on what the folks that created the program you are watching put there. We have had to turn off our surrounds a few times due to horrible production issues, not Sonos issues.

If you post what you were listening to, on which service someone might try to duplicate your experience.

You could also generate a diagnostic when the issue happens again and call Sonos support.

Hello yes thanks for the prompt response it was Cuckoo on channel 5.

I will try and generate  a diagnostic as you suggest