5. The Church has always recognized and favored the progress of the arts, admitting to the service of religion everything good and beautiful discovered by genius in the course of ages -- always, however, with due regard to the liturgical laws. Consequently modern music is also admitted to the Church, since it, too, furnishes compositions of such excellence, sobriety and gravity, that they are in no way unworthy of the liturgical functions.
Still, since modern music has risen mainly to serve profane uses, greater care must be taken with regard to it, in order that the musical compositions of modern style which are admitted in the Church may contain nothing profane, be free from reminiscences of motifs adopted in the theaters, and be not fashioned even in their external forms after the manner of profane pieces.
6. Among the different kinds of modern music, that which appears less suitable for accompanying the functions of public worship is the theatrical style, which was in the greatest vogue, especially in Italy, during the last century. This of its very nature is diametrically opposed to Gregorian Chant and classic polyphony, and therefore to the most important law of all good sacred music. Besides the intrinsic structure, the rhythm and what is known as the conventionalism of this style adapt themselves but badly to the requirements of true liturgical music.
(snip)
17. It is not permitted to have the chant preceded by long preludes or to interrupt it with intermezzo pieces.
18. The sound of the organ as an accompaniment to the chant in preludes, interludes, and the like must be not only governed by the special nature of the instrument, but must participate in all the qualities proper to sacred music as above enumerated.
19. The employment of the piano is forbidden in church, as is also that of noisy or frivolous instruments such as drums, cymbals, bells and the like.
20. It is strictly forbidden to have bands play in church, and only in special cases with the consent of the Ordinary will it be permissible to admit wind instruments, limited in number, judiciously used, and proportioned to the size of the placeprovided the composition and accompaniment be written in grave and suitable style, and conform in all respects to that proper to the organ.
(BTW, for those who say that Vatican II did away with all of that, please refer to Sacrosanctum Concillium (Constitution on the Sacred Liturgy):
…Holy Scripture, indeed, has bestowed praise upon sacred song, and the same may be said of the fathers of the Church and of the Roman pontiffs who in recent times, led by St. Pius X (speaking of the above Motu Proprio), have explained more precisely the ministerial function supplied by sacred music in the service of the Lord.
Of course, if a more current quote is desired, you can look at this quote from Benedict XVI's Spirit of the Liturgy (p 144):
...As the Church was uprooted from her Semitic soil and moved into the Greek world, a spontaneous and far-reaching fusion took place with Greek Logos mysticism, with its poetry and music, that eventually threatened to dissolve Christianity into a generalized mysticism. It was precisely hymns and their music that provided the point of entry for Gnosticism, that deadly temptation which began to subvert Christianity from within. And so it is understandable that, in their struggle for the identity of the faith and its rooting in the historical figure of Jesus Christ, the Church authorities resorted to a radical decision. The fifty-ninth canon of the Council of Laodicea forbids the use of privately composed psalms and non-canonical writings in divine worship. The fifteenth canon restricts the singing of psalms to the choir of psalm-singers, while "other people in the church should not sing."
Naturally, we should keep in mind that the Council of Laodicea was only a regional synod and so their canons do not have the force of law outside of Anatolia, but, still, there is a very good lesson in those two canons, cited by Cardinal Ratzinger.
No bells? Okay, I have a problem with that since the best Pachelbel Canon I’ve ever heard was on the bells. It was a postlude.