ARC and Samsung, according to my reading, seem to have more than usual issues.  I only have two set up so far, without incident.

One Sony with a BEAM kept switching between the BEAM and TV speakers, so I also went the toslink route.  I would do that with an ARC if I had issues.  I know that kills Atmos, but working all the time is at the top of my list. I can’t hope for some Atmos content and have dropouts constantly, someone else has to fix this, not me.

Go on the Sonos forums.  I had a customer with ARC and a Sony TV, with tremendous lip sync issues, even with a new cable box. The forum had a contributor with a checklist, and boom, one of the things he noted fixed my client’s problem.

Also run the setup for 10-15 minutes and send a diagnostic, then call Sonos.

Good luck.