Lost audio only on Apple TV app

A few days ago, I watched F1, streaming on the Apple TV service through the Apple TV app on our LG TV. Sounded & looked great (setup specs at end of post).

Later that day, I tried to watch another show on the Apple app. There was video, but no audio. Switching to other streaming apps (Netflix, Amazon, Crave) or live TV worked just fine. Going back to Apple, no audio.

I can get audio from the Apple TV app only if I use the TV internal speakers. If I turn off Simplink (it’s always been on and worked fine) the sound defaults back to the TV internal speakers, but disappears when Simplink is enabled again. 

This seems to point to something connected to Apple TV app as the issue since I haven't played with any settings for the last few years and all other inputs work as expected.

Any wisdom to offer for next steps?

Our current setup: LG 4k TV; Onkyo TX-RZ830 AVR; 500 Mbps cable connection; TP Link router; Klipsch subwoofer & small speaker system along with pair of old-school Advents. NO Apple TV box, but I'm considering it…