In general:
Music (in the overwhelming majority of cases) is already in stereo (i.e. 2.0 channels).
And pretty much all of it is loudness-maximized.
Audio on a movie is not loudness-maximized.
And if on top of that your movie comes with a surround audio track, instead of stereo, it will be even quieter when it is automatically downmixed to stereo, to suit your speaker system.
First thing you should check is that Plex player knows that you have only stereo speakers. (The subwoofer can be disregarded in a 2.1 system, as it will always derive its signal from the Left and Right channels.)
Enter the settings, section Audio and verify that it’s only set to 2.0 channels.
Don’t set the audio interface to “Automatically selected”.
With that out of the way, try DISabling “normalization when downmixing to stereo”. Compare the loudness with it on and off. But keep in mind that it will only have an effect if the selected audio track has more than 2.0 channels.
If the above measures still don’t produce enough loudness for you, you will need to employ audio dynamics compression.
Some DVD/Bluray already bring a stereo audio track which has been compressed. So if you rip your own discs, make sure to keep that stereo track and not only the 7.1 surround version.
Items recorded from TV do also have usually already various levels of compression applied.
If your movie only comes with 5.1 or (worse) 7.1 channel audio, you will need to apply dynamic compression after the downmix to stereo.
You could produce an additional stereo audio track for your files. ffmpeg can produce these for you. But you will need to mux this audio track into the original video file, because Plex cannot use audio in a separate file. [How To] Downmix Surround Sound To AAC Stereo Properly (using FFmpeg Batch Converter)
Another way would be to look for a setting in your TV or speaker system, which can activate dynamic compression.
Often it is called “Night mode” or “Audio leveller” or similar.
If none of your devices has a compressor, you can activate one in Plex HTPC.
But you will need to re-activate it again after every restart of the app: An on-demand audio dynamics compressor for PMP and Plex HTPC
