I don't think its an issues specific to the TV since I am using a Sony TV with LG soundbar. Have you tried connecting the HDMI to HDMI for sound? Maybe its just an optical cable issue with LG devices?
Can't run hdmi audio unless tv supports ARC. At least that's how I understand it.
LG web os TV and LG soundbar - sound drop/ Cutoff / issue wireless
in LG webOS Smart TV Questions
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Can't run hdmi audio unless tv supports ARC. At least that's how I understand it.