You probably will get the best result if you use a digital to analog audio converter. Amazon has a number of models:
https://www.amazon.com/DA-Converters/b?ie=UTF8&node=3224445011 . You will need to provide the optical audio cable. You connect the TV's optical audio out to the converter's input. Then use the converter's RCA output to the home theater system's audio input.
The TVs HDMI port is only an HDMI input (possibly with Audio Return Channel). This isn't usually resolved by an HDMI-RCA Converter. This converter might have worked to split off the audio signal from a Blu-ray player with an HDMI cable to the converter and then another HDMI cable to the TV and the RCA audio going to the Home Theater system.
I hope this helps.
Cindy Wells
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