Sunday Sermon, August 28 -- On music in the Liturgy.
From Musicam Sacram, the only Vatican document on sacred music since Vatican II: The parts of the Mass are divided into three categories related to singing. If anything of the second or third category is sung, then all of the first must be sung -- and it is more proper to sing the portions of the first and the second than to sing parts of the third.
The first category includes: The opening prayer, the prayer over the gifts, the prayer after communion, the preface and the Our Father (and others).
The second: The Kyrie, Gloria, Sanctus, the Creed, etc.
The third: The antiphons/hymns at opening, offertory, and communion, the readings, etc.
What this means is that, according to the law of the Church and the logic of the Mass, it is more fitting to sing the Creed than to sing an opening hymn; or again, if the "Holy, Holy, Holy" is sung, the preface must be sung.
The practice of singing hymns as the main musical theme at Mass is an example of singing AT Mass rather than singing THE Mass -- and it is an abuse that should be eliminated as soon as possible.
Listen online [here]!