We don’t need the GIRM to tell us of the importance of singing, but it’s nice to have back-up. a single section of the 1975 GIRM was expanded in 2000 into three. Two significant additions to the text were the “every care … Obligation” clause in 40a, and the quote from Sacrosanctum Concilium 116 in 41a on Gregorian chant and other kinds of sacred music.
39. 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,”[St. Augustine of Hippo, Sermo 336, 1: PL 38: 1472.] and there is also an ancient proverb: “Whoever sings well prays twice over.”
40. 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.
This is a significant strengthening of the old GIRM 19, and worded in a particular way to allow for continued growth in music-poor communities, but to advise that when music can be provided, it should be.
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.[cf. Musicam Sacram 7, 16]
41. 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.[cf. SC 116, also SC 30]
Things, from nation to nation, parish to parish, and even from one gathering of a community to another, are not equal. Unfortunately, the discussion on the place of chant gets bogged down by extremes of both ends of the spectrum, the no-chant and the all-chant camps.
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.[cf. SC 54, Inter Oecumenici 59, Musicam Sacram 47]
It’s interesting that two non-musical examples are given here, and that the Lord’s Prayer seems to be counted as a part of the Mass Ordinary. Also note that no provision is given here for choir-only music–the explicit prescription is for congregational singing.