This setup resolved audio lag for me with xfinity 4k box; 1. HDMI from cable box to tv, 2. Optical from tv to soundbar 3. Keep xfinity in dolby mode 4. Change tv to pcm 5. Set audio lag on soundbar and tv to zero . With this setup I still have dolby surround from xfinity. It seems the xfinity dolby plus tv dolby was causing the lag.
