Hello Optera,
we have been discussing this idea here ( https://forums.factorio.com/viewtopic.php?p=477446#p477446 ) and today I discover, that you already implemented it as a mod. Great!
Say, is it possible with this mod to also increase the volume for sounds above 100%? Or yould you make it possible?
The sounds that are so annoyingly faint for my taste are the battle sounds. Gun shots and explosion usually are very loud and I miss this type of "immersion".
If that is possible, then my answer to your question which sounds I would like to see volume-controllable is: All gun shot and impact/explosion sounds. From pistol to artillery. Also the train engine as of 18.5 is too quiet imo. Having louder electric miners - closer to the loudness in an industrial strip mining setup - would also raise the immersion a bit for me personally.
As of being able to turn down the volume, you already covered the offshore pump and the accu, which are th only ones I could not hear at all.
Thanks!