Weirdly, I only just had this issue with the newest update from this week. For whatever reason I still had all the old notification sounds up until just now, and I didn't skip any updates or anything. So I'm just learning about all this today, I wonder if there are others?

Mine is actually really loud but the rebound sound scares the shit out of me!! It sounds like a ghost, or like a horror movie special effect lol. It's too spooky that it can't be changed :( Hopefully I'll get used to it but what a weird thing to lock out of customizing. Does apple want this to be their iconic sound or something..?