I have a similar problem I have changed the STB & the HDMI cable, so I know they are fine, Samsung hu8500 tv. Sonos Beam and 5.1 system. Xfinity box connected via hdmi to tv. ARC on tv to Sonos beam. Apple tv to tv. Apple tv gives 5.1 dolby. Xfinity box shows only sterio on the auto select screen. Beam has HDMI to connect to tv & optical imput. Seems to me there is somesort of handshack issue between Xfinity box & samsung tv. Has anyone solved this, or is there additional advice.
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