I have a vintage NAD amp with B&W speakers and like to play the TV audio through my HiFi set up using the optical audio out and converting the digital signal to analogue using a NEET DAC. It all works really well except when I play Netflix from the smart TV when I get strong interference on the audio channel, it plays fine on the TV's built-in speakers. Also, if I cast Netflix from my iPhone X the audio plays perfectly through my amp set up.
I've just solved it myself, didn't realise I had access to this via the TV remote but found and switched the audio output to PCM and turned off the TV speakers, works like a charm
You're welcome if that helps anyone else