This is the motu proprio of Pope St. Pius X on Sacred Music. While some of the regulations (and definitely most of the attitudes) on this subject have changed in the nearly 100 years since he wrote it, the reasoning behind it is still very much relevant (Good Theology doesn't go out of style, one would think--it merely gets ignored by generations of progressive "reformers" who want things their way).
Particularly poignant is this quote:
On these grounds Gregorian Chant has always been regarded as the suprememodel for sacred music, so that it is fully legitimate to lay down thefollowing rule: the more closely a composition for church approaches in its movement, inspiration and savor the Gregorian form, the more sacred and liturgical it becomes; and the more out of harmony it is with that supreme model, the less worthy it is of the temple.
The ancient traditional Gregorian Chant must, therefore, in a large measure be restored to the functions of public worship, and the fact must be accepted by all that an ecclesiastical function loses none of its solemnity when accompanied by this music alone.
An interesting read, to be sure--more to follow on liturgy when I finish Spirit of the Liturgy.