The Christian faithful who come together as one in expectation of the Lord’s coming are instructed by the Apostle Paul to sing together Psalms, hymns, and spiritual canticles (cf. Col 3:16). Singing is the sign of the heart’s joy (cf. Acts 2:46). Thus St. Augustine says rightly, “Singing is for one who loves,”[48] and there is also an ancient proverb: “Whoever sings well prays twice over.”
Great importance should therefore be attached to the use of singing in the celebration of the Mass, with due consideration for the culture of peoples and abilities of each liturgical assembly. Although it is not always necessary (e.g., in weekday Masses) to sing all the texts that are in principle meant to be sung, every care should be taken that singing by the ministers and the people not be absent in celebrations that occur on Sundays and on Holydays of Obligation.
However, in the choosing of the parts actually to be sung, preference is to be given to those that are of greater importance and especially to those which are to be sung by the Priest or the Deacon or a reader, with the people replying, or by the Priest and people together.[49]
The main place should be given, all things being equal, to Gregorian chant, as being proper to the Roman Liturgy. Other kinds of sacred music, in particular polyphony, are in no way excluded, provided that they correspond to the spirit of the liturgical action and that they foster the participation of all the faithful.[50]
Since the faithful from different countries come
together ever more frequently, it is desirable that they know how to
sing together at least some parts of the Ordinary of the Mass in Latin,
especially the Profession of Faith and the Lord’s Prayer, according to
the simpler settings.[51]
[SOURCE: http://www.usccb.org/prayer-and-worship/the-mass/general-instruction-of-the-roman-missal/girm-chapter-2.cfm]
