From the General Instruction of the Roman Missal:
“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 the people 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 inot 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. [footnote 49: Cf. Sacred Congregation of Rites, Instruction, Musicam sacram , 5 March 1967, nos. 7, 16: …].”
The footnote refers to
“7. Between the solemn, fuller form of liturgical celebration, in which everything that demands singing is in fact sung, and the simplest form, in which singing is not used, there can be various degrees according to the greater or lesser place allotted to singing. However, in selecting the parts which are to be sung, one should start with those that are by their nature of greater importance, and especially those which are to be sung by the priest or by the ministers, with the people replying, or those which are to be sung by the priest and people together. The other parts may be gradually added according as they are proper to the people alone or to the choir alone.”
“16. One cannot find anything more religious and more joyful in sacred celebrations than a whole congregation expressing its faith and devotion in song. Therefore the active participation of the whole people, which is shown in singing, is to be carefully promoted as follows:
(a) It should first of all include acclamations, responses to the greetings of the priest and ministers and to the prayers of litany form, and also antiphons and psalms, refrains or repeated responses, hymns and canticles.
(b) Through suitable instruction and practices, the people should be gradually led to a fuller—indeed, to a complete—participation in those parts of the singing which pertain to them.
( c) Some of the people’s song, however, especially if the faithful have not yet been sufficiently instructed, or if musical settings for several voices are used, can be handed over to the choir alone, provided that the people are not excluded from those parts that concern them. But the usage of entrusting to the choir alone the entire singing of the whole Proper and of the whole Ordinary, to the complete exclusion of the people’s participation in the singing, is to be deprecated.”
The full document is at http://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_instr_19670305_musicam-sacram_en.html .
[Excerpts from The Roman Missal © 2010 ICEL. All rights reserved.]