Okay, using my BB headcanons, Razz does not have the same relationship with Alphys as Edge does with his Undyne. Razz’s relationship was much more distant and reserved. The queen was always watching, and both of them were afraid to displease her by getting attached, so he wouldn’t necessarily have had cooking or music lessons.
I LOVE the idea of Slim playing an instrument—and playing it well—but I don’t know what he would have the opportunity to learn. But, yes, Slim and Paps are both musically inclined, though it’s more exaggerated in Slim. I like the idea of him being able to sing, of course, but a musical instrument seems so appropriate for him.
(Seriously, though, Paps can dance like nobody’s business when he’s properly motivated.)
Maybe Slim found a busted up instrument in the dump and taught himself and hid it until it was discovered and destroyed; maybe he mimicked NTT’s music on the TV and taught himself an instrument that way; maybe if he had no opportunity to play any instruments (or even if he did but they were broken or destroyed so he learned that he couldn’t risk hiding any more) then he taught himself to sing by just practicing whenever he was alone bc it filled the silence and it made him happy and maybe it made his baby brother smile when he was an infant and Slim’s heart swells with music and he just has to hide all of his happiness or else it is taken away.
(Also idk yet if his voice is merely rarely used or if his throat/neck was actually damaged somehow, but if his threat was damaged and there weren’t an established reason for it, it could be because he was caught singing or humming twice and thus punished harshly for disobedience)
“He just has to hide all of his happiness, or it’s taken away.”—That is so sad and poignant and depressingly true. Cripes.
(I have so many Slim feelings right now guys, you don’t even know. I just want to wrap him up in a blanket burrito and carry him off.)