Looking at the tv manual pg 21 says about digital audio output:
Digital Audio Out — Changes the type of processing for digital audio out and HDMI ARC output when connected to a home theater audio system . Select Auto, PCM, Dolby D or Bitstream .- To hear talk back when Talk Back function is enabled, digital audio out must be set to PCM .•
TIP: You must select Bitstream for audio with more than two channels (e .g ., 3 .0, 5 .0, or 5 .1) .
Was wondering what settings other than Dolby D you've tried now. Wonder what Talk Back is and also that it doesn't even mention optical....