Hmm I took a quick look in Sonos app cause use Sonos Beam and it shows Stereo PCM… but I am also hearing sound come from rear speakers and for sure never heard sound from rears when using Roku Ultra and Xfinity app…. So not sure what has changed.
Again, we may be being fooled here but I'm not convinced yet that DD5.1 is unavailable. Knowing Comcast, probably. But still, my ears have some subjective hope.
Did you try something that you KNOW is a DD5.1 source? Spiderman Homecoming was running on (was it) Starz HD but may be done by now. Look through the guide for a recent movie or show you know is in DD5.1 and check again. I'd love to know conclusively myself.
Edit: My post made me wonder about going ahead and looking at the PrimeTime schedule to see if shows I know use 5.1 show anything different in the descriptions. While I don't see this in some that I know are DD shows, there is this D icon in others (like tonights Law & Order) I've boxed in Red below that MIGHT mean something (good)...
...or might mean who knows what else. With Comcast, it might mean "Debit card required" or "Deducting all we can from you if you watch this" or similar.
However, it's not hard to guess that that might also mean at least "Dolby"... possibly only Stereo but maybe more.
I couldn't find anything in a bunch of checks that has that showing in descriptions right now. But I didn't check every possible channel so maybe someone will find something and can give it a try with something that will tell them for sure what kind of sound they are getting.
I don't want to get anyone's hopes up much. Streaming is usually NOT DD5.1 audio and that would definitely differentiate this option vs. leased boxes for those that want true surround sound... one reason I go the Channels (app) plus HD HomeRunPrime route myself. But at least it's a maybe hypothesis right now until someone can confirm or refute for certain.