I’m having the same problem.  I previously had Arc+300s+Sub Gen 3, and it all worked fine.  After upgrading to the Arc Ultra and adding a Sub Gen 4, I get intermittent brief audio drops.

As an example, in Endgame, when Cap says “Anyone see an ugly brown van out there?” it came across as “Anyone see an ugly brown -an out there?”

There are some Samsung TV owners wondering about the same in Reddit. Someone wrote that the drop outs stopped when they replaced the Sonos-provided hdmi cable with a different one. They didn’t mention which brand and what cable exactly, though. 

Change my configuration last night, using an Arcana to split the audio and send it straight to the ultra.

Change my configuration last night, using an Arcana to split the audio and send it straight to the ultra. Not heard the audio drop yet

So that suggests that it’s the audio being passed through the Samsung that is the issue

my sources are an Apple TV and Nvidia shield pro. 

I changed from the Sonos provided cable, to a 4K compatible high-speed alternative. Only had a chance to try it for 20 minutes, but didn’t hear sound dropout.

Will try extensively over the next few days and see if that makes the difference

Interesting - I’ve been using the cable provided by Sonos, curious to hear how that goes.  I’ll order a high speed cable as well, they’re not that expensive.

I’ve heard audio drops with a different cable. So no closer to a resolution

It is unfortunate the cable didn’t solve it. Maybe it’s something in the way the Samsung pass-through behaves with Sonos. This issue is preventing me from buying the Arc Ultra until I buy a different brand TV.

Contacted Sonos support this afternoon. Very impressed with their level of knowledge.  Ran diagnostics and they could see the problem. 
First action was to try a different cable to the soundbar.  I tried the one that I’d used with my Arc.  The problem continued. 

On my second call, the Support analyst I spoke to said that this had been reported several times already. And their recommendation was to turn off CEC; that the Arc ultra Is more sensitive than the arc to competing CEc messages. If turning it off doesn’t work they recommended an HDMI CEC stripper on my inputs. I have 3 on order from Amazon

I ordered a new highspeed HDMI cable, and since swapping that cable out, I haven’t had any drops at all after about 6 hours of use.

It seems really strange to me that a cable would be an issue with a digital signal, so I wonder if it has something to do with just unplugging the cable in some cases. (Although that clearly didn’t fix it for you).

For reference, I have a S90C.  I’ll update if the audio drops start up again.

Three HDMI CEC strippers fitted to my HDMI ports.  All sounding good

I have used a Sinox 8K Ultra High speed HDMI and this solved the issue for me. Just tested it in my showroom.

Just coming here to say this isn't a Samsung only thing. Having the same issue with my Sony TV. I'm using the same cable as my Arc (didn't feel like swapping). Will try swapping cables but this is kinda ridiculous