Further update: the optical cable solved the sound issue but something else incredibly irritating has reared its head. Every time I rewind the BR player and press play, the AVR changes mode to TV Audio (ARC). This happens on every occasion without fail. I suspect I need to fiddle with the TV's settings.
To add to the original problem: the entire issue, including the update in this post, never occurred with the Samsung LED TV I used previously. In fact, the problem never occurred with the LG OLED before, say, two months ago.
I'm still hugely puzzled and am open to any ideas and help from anyone at this point.