If you have a cable box and run HDMI to the TV, most TVs will not pass more than stereo. To get 5.1 HD sound you run from the source to the soundbar or use apps in the TV.
This does not help folk that want 5.1 sound from the TV when the source is your cable or Apple TV boxes. And getting stereo sound is not the same menu so I can't guide you past "set audio to stereo, PCM and re-test."
I see this bar has HDMI inputs and outputs so it can behave like a receiver (but it is not a receiver.)
So my setup would be from cable box (HDMI all the way) to the sound bar then hdmi from soundbar to TV.
I cannot get my LG SK10Y 5.1.2 Soundbar set up to work with a cable box and Apple TV 4K. LG support has been zero help in this. Below is the response I received from them when I asked them the same question.
Dear Phillip John,
Thank you for contacting LG regarding your SK10Y.
The SK10Y offers both an optical and HDMI input to allow you to connect to your receiver.
Thank you for choosing LG
LG Customer Service