Play any video. While the video player is playing press the * button on your remote. Look for Volume Mode in the options menu that appears. Make sure Volume Mode is OFF.
If it is LEVELING it will make loud sounds quiet and quiet sounds a little louder. It basically makes all audio the same sound level.
If it is NIGHT it will make loud sounds quiet and quiet sounds just stay quiet. It basically makes all loud sounds quieter.
When Volume Mode is not OFF it will only use stereo. It is decimate the audio. When you turn Volume Mode to OFF you allow the normal sound levels.
Even though I did not ask the question and I do not have the problem described I want to thank you for posting this. Since the "modes" were introduced I tried to get Roku, on their help board, to explain exactly what the various modes did and how they were expected to work but the only answers I received were so convoluted and ambiguous and often contradicted each other that I basically was less informed after reading them than I was before.
You just gave a simple, direct and understandable answer in just a few concise and direct sentences to the question that seems to be beyond Roku's ability to answer.
I had made a guess based on observation and it was. at least mostly, correct but it is good to have confirmation.
Once again, thank you very much.
BTW: I use nothing but stereo audio so I keep the setting at "Leveling" so I can hear soft sounds and mostly avoid being blasted out of my chair on loud sections.