My Samsung TV has issues with external audio at times, or even when playing local media that isn't using AC-3/DD. My solution long ago was to use an AVR to handle all of the audio, and connect all my devices directly to the AVR.
Your Onkyo has four HDMI inputs, all capable of 4K/60. Use them, and don't connect anything directly to the TV. You don't need ARC if you connect your devices directly to the AVR.
Dan
Roku Community Streaming Expert
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