"Absolutely direct experience of the will is impossible, because it will always be mediated by time, but in first-personal experience of volition and the experience of music the thing in itself is no longer veiled by our other forms of cognitive conditioning."
There's also a less poetic, but I believe a more accurate definition in the blog you've linked:
"Music is a hidden arithmetic exercise of the soul, which doesn't know that it is counting." (Gottfried Leibniz)
We only know what the word "music" means from our own subjective experience of listening to specific examples of music.
At most we can observe common features of musical items, and include a description of those features in a definition. But such a definition will fall way short of accurately separating what is music from what is not music.
There are other subjective phenomena that we have words for, but for most subjective phenomena we have _some_ objective understanding of what is going on.
Music seems to win the prize for being both very familiar _and_ very mysterious.