Audio becomes overly quiet or loud at various times while watching movie

    I need some help fixing my audio issues before my family kicks me out of the house. :)

    When we watch movies, the sounds will often get very quiet, and then a bit later it will return to normal or get very loud. The timing of the audio getting quiet/loud is repeatable within the same movie. Meaning if I watch the same part of the movie, the sound will get quiet/loud at the same time in the movie. The timing of the disruptions is not on a fixed interval, it could happen 1 minute apart or 15 minutes apart or whatever, but as I said it's reproducible. It seems to be somewhat related to the movie itself, like a scene (not chapter) change. If I watch the same movie with the USB drive plugged into my Sony DVD player which is connected via HDMI to the TV, the audio remains good watching the same scenes I have the problems with via LibreELEC.

    My Setup:

    My movies are in m4v format and on an external USB drive

    LibreELEC 9.2.1 running on a Raspberry Pi 4 (With a Pi4 recommended 5.1v 3A power supply)

    The Pi is connected via HDMI to my Vizio E480i-B2 TV

    The TV is then connected via optical cable to my Vizio S3851w-D4 sound bar and 5.1 speaker system.

    The Vizio system has surround sound, and supports DTS. It also has settings for "Night Mode" and "TruVolume". I mostly have been testing with these off. But, I mention these as these settings seem to also automatically adjust audio.

    I have fiddled with a ton of the audio settings in Kodi including the PC Speakers 5.1 and AVR HDMI (no HD Audio) setups mentioned here: Audio quickstart guide - Official Kodi Wiki. I have also fiddled with Kodi settings in conjunction with my speaker settings. But I have not been able to crack this one.

    I also tried hooking the Pi right to the sound bar via the speaker/headphone/mini jack on the Pi to the soundbar's AUX (1) 2 port audio in. This did not solve the problem.

    I one time noticed in one movie, that by setting pass-through on I could not adjust the volume through Kodi (I assume this is by design) and I did not have the audio problem. But then watching a different movie, I experienced the usual audio problems (and could adjust Kodi volume) even though I had not changed anything. Like I said, I've tried a lot of different settings and read lots of posts in different forums and have not figured it out.

    Any help would be greatly appreciated.

    Thank you!

    Edited 2 times, last by opendavele: Update details of setup and some of the things I've changed. (April 4, 2020 at 3:47 PM).

    grael my system was already setup with downmix disabled. I just ran the same test segment of movie and it did not change with downmix enabled or disabled. But thank you for the suggestion.

    Edited once, last by opendavele (April 5, 2020 at 4:22 PM).