So, I was going through the game files and found out that every in-game sound is specified in a script. It looks like this:

audio\sounds\wetrainloop.wav /2D /LOOP /FREQ=22050 /VOL=-900

If I change the VOL to lower values, it fixes the annoyingly loud rain sound on PC version, which probably cannot be fixed in any other way (apart from modifying the audio file itself, but modifying in script is way easier). I'm aware this is modifying the game files, but it doesn't change anything gameplaywise, only volume of certain sounds like rain or "Loose the tail, man" voices.

Are you guys okay with it?