Can this be one of my acceptable answers for "why do you want to study a PhD"?
When I ask the question "why do you want to study towards a PhD?", the very last thing I want to hear is something entirely unrelated to studying towards a PhD. Not because I fear that you get sidetracked (I don't necessarily worry much about that), and certainly not because I have some inherent bias against piano players, but because I worry that you actually don't know why you want to do a PhD. This is true for a surprisingly large percentage of applicants, and typically a fairly bad sign.

Consequently, if you get this question, I suggest that you explain why you want to do a PhD, and nothing else. If the question is why you want to do a PhD at exactly this university, this can be part of your answer, but it should not be the only, or even the main, answer to that question either. Quite frankly I would imagine this to be largely ignored - playing piano is not memorable enough to make you stick out from the applicant pool, and unrelated enough to virtually all fields of research that I won't count particularly positively either.