Symptom: On the Apple TV 4K device, every single app plays normally (Netflix, Prime, HBO, Disney+, Paramount+, etc.). Except: The Apple TV+ app plays video with music and sound effects, but no dialog.
Clearly not about the Apple device.
What is it about the Apple TV APP that is different from every other app streaming on the Apple TV device?
HDMI Setup: Apple TV 4K —> Denon Receiver —> Hisense 8K U8N (2024)
Late reply: I experienced something like this when my audio output chain of devices was getting data it could not decode.
Apple says (in Change audio settings on Apple TV):
If you’re experiencing playback problems with equipment that only supports specific audio formats, select Change Format and choose either Dolby Digital 5.1 or Stereo.
So try setting the Audio output format explicitly (Settings > Video and Audio > Audio Format) and see if the audio reappears.
Then at least you know it’s a format compatibility issue.