Mostly just to back up what others have said and give a bit more background.  Initially, the idea was simply to "refresh" the sounds, much like we have also done with the logo in 2017.3.  It wasn't that nobody liked them, but it was really split - there were people who preferred the old sounds, some who liked the new sounds, and some who wanted to change the sounds, but didn't like the new ones either.  We hadn't considered that until the feedback started coming in from the snapshot users.  We realised then that what we needed to implement, more than just a new set of sounds on its own, was a way for people to be able to change their own sounds.  One idea for this is along the lines of the way you can install add-ons.  Ideally, unlike add-ons, we want something that will be a bit easier for users to package without needing programming skills.