We had a similar problem when the dialogue stopped, but ambient audio kept working on some recorded dramas from NPR via Oregon Public Broadcasting (OPB) one just one of our two receivers.
After tearing out much of whatever hair I have left, I noticed that "Video Description" had accidentally been turned on.
When I turned it off, the dialog on the problem shows went back to normal.
It seems illogical that video description should remove dialogue and that's probably a bug, but we're back to hearing dialogue.
What is video description? (From Xfinity:) Video description refers to audio-narrated descriptions of key visual elements of programming inserted within natural pauses in dialogue. Video description makes television programs, feature films and other media accessible to people who are blind or have low vision or a visual disability, creating a richer, more independent experience. For example, video description describes key visual elements such as actions, facial expressions, costumes or scene changes in a program that a viewer who is blind or has low vision or a visual disability would otherwise miss.
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