5.1

@BIslander​ DVD box, two Wi-Fi boxes and one smaller box all the same expert mode does not produce Dolby 5.1 only stereo.

@andyross​ yup but still does not work if you look at the sound comming in to tv just says pcm and Dolby or Dolby+ are whited out and not available, but you switch to Netflix or prime and they are and you can here the difference 

That post makes it sound like the Xfinity box is connected to the TV rather than the sound system.   If so, that could be the problem as some sets are limited to stereo PCM from external devices.  If so, have you tried attaching the Xfinity box to your sound processor to see if it gives 5.1 that way? 

One box goes into a AV receiver one box goes directly in the TV to Wi-Fi boxes go directly into the TV. None of the TV show 5.1 no matter how they’re hooked up so it’s got nothing to do with the processor it’s all about the signal that Comcast sent me I do not see 5.1 on a single TV, when you go to the sound settings it’s all you get is PCM .Dolby digital is white it out unless you go directly to a streaming service like prime Netflix YouTube then the TV shows 5.1 Dolby digital, PCM is an option but you choose Dolby digital in that case, and the sound is obviously much better . 

TVs are stereo devices so the boxes attached to TV sets are not going to output DD 5.1.  So, concentrate on the one box connected to an AVR. 

I am a bit confused by your description.  It sounds like you are saying the box setting itself changes from stereo PCM to DD 5.1 depending on what source you are watching.  But, perhaps I don't understand as that is not the way any of my Comcast boxes have ever worked.  And, again, you post describes the format reported by the TV, not the AVR or the Comcast box.  

What model box is connected to the AVR?   How is it connected to the AVR?  And what are the audio output settings?