Stream 5.1 Audio

I'm giving Xfinity Stream a try and I am only getting a stereo (PCM 2.0) output from content on the app on both a Fire Cube 3 and Roku Ultra.  That's true with any program I try.  The devices do 5.1 and Atmos just fine with other apps like Netflix.  I don't see any settings on the Xfinity apps for audio output.

Does Stream actually do surround sound?   If so, is it limited to particular platforms and which would those be?

Thanks