The Musical Shape of the Liturgy

The reforms of the liturgy resulting from the Second Vatican Council have greatly increased the freedom of choice of liturgical music; the council also encouraged the composition of new music for the sacred liturgy. However, every freedom entails a corresponding responsibility; and it does not seem that, in the years since the council, the responsibility for the choice of sacred music has been exercised with equal wisdom in all circles. To judge by what is normally heard in the churches, one might even conclude that the Church no longer holds any standards in the realm of sacred music, and that, in fact, anything goes.
The council did not leave all up in the air, however, and if its Constitution on the Sacred Liturgy had been seriously heeded, a living tradition would still be alive everywhere, and we would have added musical works of some permanence to the “store of treasures” of sacred music. The council laid down some rather specific norms which can serve as a basis for developing an understanding of sacred music and thus for choosing wisely.
In its chapter on sacred music, the council declared that the solemn sung form of the liturgy is the higher form, that of all the arts music represents the greatest store of traditional treasures of the liturgy, that music is the more holy insofar as it is intimately connected to the liturgical action, and that Gregorian chant is the normative music of the Roman rite. Moreover, in speaking of innovations in general, it required that new forms derive organically from existing ones.

From Sacred Music 102, no. 3 (1975), reprinted in The Musical Shape of the Liturgy by William Peter Mahrt (2012)