It seems to be something that is better solved by having the player normalize the audio at playback, so I think you're better off asking Jellyfin users how they do it. I don't know anything about Jellyfin, but a quick search shows that it does have separate options for music loudness and video loudness.

If you insist on editing the files themselves you could use something like Audacity to (destructively) nornalize the files to the same loudness by LUFS.