I have not had a problem with this until today worked as early as last night even. I screen share my mac through my apple tv and today the sound doesnt come out of my soundbar that is connected to my tv like it usually does and only plays through my computer speakers. I reset everything and it plays through other sites and computer programs like it's supposed to. It's just the xfinity stream site that this no longer working properly.
I am having the same issue. Video going to my TV, but sound only coming from laptop. I called Xfinity and they said it was an issue on my end, not theirs. Aparently they dont read these forums since others are having the same issue. I hope they fix it soon.....soon like today.