I've poked around online and it seems like Jellyfin had (music) volume normalization added to it sometime recently. However, I'm struggling to verify that it's enabled/working. Is it something I have to enable or is it on by default? If it's on shouldn't I be able to see something like a LUFS or ReplyGain value in each song's metadata?
UPDATE:
I'm not familiar with Jellyfin's git strategy, but it seems like even though the audio normalization has made it into the master branch it has NOT made it into the 10.8.z release tag/branch. I determined this by looking for the changes in Emby.Server.Implementations/Data/SqliteItemRepository.cs from the normalization PR in the current version of the file in 10.8.z and they were not present.
Does volume normalization mean that we can choose the volume level finally? I cannot figure out a way to do this in 10.8.x
Jellyfin, across every single device that I have, is literally 50%+ quieter than every other app. I have to double the volume on my TV w/Chromecast for example to watch a show.
It's specific to music, per the OP. As far as boosting the volume goes, it varies based on your client and devices, but on many TVs, sound bars, and AVRs, you can enable "Night Mode" and it'll turn on a compressor that basically makes all content played on said device sound closer together in volume (so less of a dramatic swing between Jellyfin volume and other stuff)
It won't. In fact, it might even make that part worse because the quieter parts would become even quieter.
What you need here is a "midnight mode" which is just a compressor; it reduces the dynamic range. Since dynamic range is an aspect of audio quality, this is not something you generally want.
Gain normalisation just ensures that different audio tracks are, on "average", the same volume so that you don't have to change volume all the time to accommodate the different mix of each song.
Spotify has these features for example under it's "Normalise volume" setting; the first two settings do gain normalisation and the high setting also adds a compressor I believe.