I know you have a lot to do with these comments. But it would help if someone would just State at the beginning of the comments what case the sentence is in.
I'd love to be able to just 'pin' a comment at the top, but I can't :/
"Ona" (the subject -> Nominative) + "słucha" (3rd person singular verb) + "smutnej muzyki" (Genitive, as needed by the verb "słuchać").
Is this genetive?
Yes it is.