The weird things is, if you click sing in the shower, they usually don't actually sing (though they get the skill gain). But if you just have them take a regular shower, they usually do if they are in a decent mood. There's a "sing in the shower" potential animation outcome that is in the regular shower, but a different sing in the shower animation choice in the coding that doesn't seem to work great. Like I said, weird, and I swapped it in my game.