the sound is very soft

[Tim]

Usually when audio is down-mixed, aside from reducing the tracks, it also loses a lot of volume. Perhaps the audio is down-mixed without there being a gain applied to it. Like @TheDreadPirate mentioned, you can apply the gain on Jellyfin's side only when Jellyfin transcodes, but if it direct plays, another option would be to increase the volume yourself in the source file. This can be done by encoding and down-mixing (if that was the issue) the audio track again. When I down-mix an audio track (i.e., 5.1 -> stereo), I tend to apply a gain of at minimum 2 to the track when encoding it.

What i said only applies if it is actually down-mixing, which could very well not be the issue here.