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!

Thanks grael, I had read your reply on that post the other day, but let me try it again. I'll report back.

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.

I still have not found a solution. I am happy to provide more information, logs, whatever if people think it will help.

Let me know,

Thanks!