“Music is the art of sounds in the movement of time.”

This is Your Brain on Music (2006)
Context: It is only in the last five hundred years that music has become a spectator activity—the thought of a musical concert in which a class of "experts" performed for an appreciative audience was virtually unkown throughout our history as a species. And it has only been in the last hundred years or so that the ties between musical sound and human movement have been minimized.

The Liberation of Sound: An Introduction to Electronic Music (Prentice-Hall edition, 1972)

The History of the Worthies of England (1662): Musicians.

"The Phantom of Kansas" (1976), The World Treasury of Science Fiction (ed. David Hartwell), p. 375

Paraphrased and misattributed, actually from "Die Musik des neunzehnten Jahrhunderts und ihre Pflege: Methode der Musik" ("The Music of the Nineteenth Century, and its Culture") by Adolf Bernhard Marx: "Die Kunst ist stets und überall das geheime Bekenntnis und unsterbliche Denkmal ihrer Zeit." ("Art is always and everywhere the secret confession as well as the undying monuments of its time.").
Misattributed

Henry Flynt: "Essay: Concept Art." (1961) In: La Monte Young (ed.) An Anthology, 1963.

The Usurpation Of Language (1910)

Peter Quince at the Clavier (1915)
Context: Just as my fingers on these keys Make music, so the self-same sounds On my spirit make a music, too. Music is feeling, then, not sound;
And thus it is that what I feel,
Here in this room, desiring you,
Thinking of your blue-shadowed silk,
Is music.