Had the same problem with my 55UB8500, fixed it by turning off "Auto Power Sync" in the Simplink menu. Your speaker system won't shut off when you turn off you tv, but it may have an auto shutoff, like my lg surround system does, and when you turn your tv back on it will turn on or wake up your sound system and connect with arc.
HDMI ARC To Soundbar not working when turning on each morning
in LG TV and LG webOS Smart TV Discussion
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Had the same problem with my 55UB8500, fixed it by turning off "Auto Power Sync" in the Simplink menu. Your speaker system won't shut off when you turn off you tv, but it may have an auto shutoff, like my lg surround system does, and when you turn your tv back on it will turn on or wake up your sound system and connect with arc.