Not quite -- the noun cerddoriaeth is feminine and so it receives soft mutation after the definite article y, yr, 'r as well. (And would receive soft mutation after the indefinite un as well, if it made sense to speak of "a music".)
So it would be gwrando ar y gerddoriaeth.
Dw i'n mwynhau chwarae gerddoriaeth / chwarae caneuon / chwarae piano / dysgu gerddoriaeth newydd...Am I on the right track? Can you suggest other useful expressions? (I don't care so much right now about the mutations, thanks, just want to get the basics of structures with music.)