Windows has a volume mixer which allows you to balance the sound levels of the various apps with each other. Google it to find out how to invoke it.
If that is not enough, follow these hints:
- check the speaker configuration in Windows. If there are only two speakers connected, set it to Stereo. Don’t set it to more, like 5.1 or 7.1
- don’t pick a 5.1 or even 7.1 audio track if you want to play the video on a device which only has one or two speakers
- if there is no stereo audio track, mixing it down to stereo will inevitably lead to quietening of the overall levels. This is done to prevent digital clipping. To reduce that effect, you need to have a loudness compressor. Plex currently doesn’t have one available.
- Windows has a “loudness leveling” effect which you can enable in the sound settings. Keep in mind that this is quite crude and only recommended for speech content. Music will be negatively affected by it.
- if you can get used to controlling your player with a keyboard instead of a mouse, you can use Plex HTPC instead of the Plex web app. The former can be extended with an on-demand loudness compressor: An on-demand audio dynamics compressor for PMP and Plex HTPC
- you can add a stereo audio track to your video, which is optimized for intelligibility by using [How To] Downmix Surround Sound To AAC Stereo Properly (using FFmpeg Batch Converter)
