It doesn’t sound like mracicot is encoding his media incorrectly
The levels and surround sound work fine, it’s only in Plex/Chrome that he’s having a problem
A browser is the least capable client so any time I hear about a playback issue like this, the browser becomes my main suspect
Windows 
Chrome 
hmmm 
If you’re on Windows
Google Chrome - How to Enable 5.1 Channel Surround Sound - Windows 10
https://appuals.com/how-to-configure-and-test-5-1-surround-sound-on-windows-10/
I’m curious if the audio is transcoding in Plex when you play it in chrome
Do you have another android client like a Firestick or Nvidia Shield you can test this on?
Besides all that, low audio in Plex is a known issue for some people and nobody seems to know why
I haven’t read most of that thread but it seems to revolve around 5.1 for some reason.
There is a few people in there that found solutions for their specific problem but this doesn’t appear to be a one size fits all situation. Maybe you can try some of the things they mentioned though
Good Luck
