The British mathematician Alan Turing posed a test (the Turing Test) to determine whether a computer was as smart as a human. It was simply that if you interact with a computer and you can't tell if it is a computer or a human being responding then the computer was is as smart as a human. In the case of music, it you can't tell the difference between artificially generated music and human generated music then the artificially generated music is as good as that created by a human.
A more general question is whether humans can, in fact, do anything that a computer cannot do. The mathematician Kurt Godel "proved" that math is incomplete, i.e., that there are some mathematical truths the cannot be deduced mathematically. These truths are based on self reference, i.e., statements that talk about themselves. I won't be so arrogant as to attempt to explain that, but there is an interesting book by Nobel laureate Roger Penrose, which goes into it in exquisite detail. The book is: The Emperor's New Mind. In this light, it is interesting to note that Bach's music is recursive in a kind of self-referential way. It repeats itself in different ways at different points in time. Maybe that's an example of music AI can't come up with.