TV Repair Questions? Ask a TV Technician for Answers ASAP
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On the three newer LG sets that I've setup, the Optical port was the only one that allowed built in streaming apps to produce sound through an external AV receiver. The HDMI ARC only processed signals input to the TV via HDMI in the first place, and all other sources were output via optical. In the TV's setup MENU, please try selecting optical and then use an optical cable from the TV to the Onkyo and let me know if that works.
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Thanks for the reply, and I agree. I'm glad all is working out!