I have been using the Plex for Kodi for a while now but I have noticed that many of my movies the audio is so quiet and there is no way to increase the volume. I'm using a 7.1 bluetooth surround sound bar and all my movies have either 5.1 or 7.1 audio. It appears the Plex is downmixing the audio to stereo and won't play the 5.1 or 7.1 audio. If I play the movies from the web app or local files through Kodi audio is perfectly fine so there is something going on with the Plex app for Kodi. I have tried a different Plex app for Kodi and it appears to work just fine except it has a lot of other bugs that the official Plex app doesn't. Anyone have any suggestions?
The Plex Web App and Kodi should be playing the files directly, so that would explain why they're working.
Plex will transcode files based on the client settings in an attempt to deliver compatible streams. Dig into your client settings to see if it's set for something else, and make sure "direct streams" are enabled if available.
I don't have the app, so I don't know where the settings would be precisely... but the Plex principles should still apply.
Kodi has the ability to do volume amplification across all media, in the Sound settings.