When your setup contains a TV, a set top box, and a home theater, the
app assumes that the audio is piped through the home theater setup, and not through your TV or set top box.
If you still use either your TV or set top box for volume control, then you need to reconfigure the volume keys to control the correct device in your room setup.
TV app, and then tap
Select the volume keys, and then follow the onscreen instructions to reconfigure the volume control.