From the General Instruction of the Roman Missal, about the Gloria:
“53. … The text of this hymn may not be replaced by any other. It is intoned by the Priest or, if appropriate, by a cantor or by the choir; but it is sung either by everyone together, or by the people alternately with the choir, or by the choir alone. If not sung, it is to be recited either by everybody together or by two choirs responding one to the other.
It is sung or said on Sundays outside Advent and Lent, and also on Solemnities and Feasts, and at particular celebrations of a more solemn character.”
(From: Roman Missal, English translation according to the third typical edition, approved for use in the dioceses of Australia, England and Wales, and Scotland, published by Catholic Truth Society, in 2010, ISBN 9781860827303, page 42.)
From the Ceremonial of Bishops, Chapter 1 “Stational Mass of the Diocesan Bishop”:
“120 The form of the stational Mass should be retained especially on the greater solemnities of the liturgical year, on Holy Thursday for the blessing of the chrism and the Mass of the Lords Supper, at the celebration honoring the founder of the local Church or the patron of the diocese, on the anniversary of the bishop’s episcopal ordination, at large gatherings of the Christian people, and during pastoral visitations.
121 The stational Mass should be a sung Mass, in accord with the provisions of the General Instruction of the Roman Missal.
[footnote 4: See GIRM, nos. 12, 18, 19, 77, 313: DOL 208, nos. 1402, 1408, 1409, 1467, 1703. Where applicable, the Ordo cantus Missae should be followed. See Missale Romanum, Ordo cantus Missae (Vatican Polyglot Press, 1972), Introduction: DOL 535, nos. 4276-4302. See MS, nos. 7, 16, 29-31: AAS 59 (1967), pp. 302, 305, 308-309; DOL 508, nos. 4128, 4137, 4150, 4151.”
(From: Ceremonial of Bishops, Liturgical Press, 1989, ISBN 0-8146-1818-9, page 49.)
To explain the abbreviations: “DOL” is the book Documents on the Liturgy 1963-1979, Liturgical Press, Minnesota, 1982, ISBN 0-8146-1281-4.
“MS” is the Instruction Musicam sacram, 5 March 1967, of the Sacred Congregation of Rites.
From the 1967 Instruction *Musicam sacram, (from DOL):
“7. The amount of singing determines the gradations between the most solemn form of liturgical celebrations, in which all the parts calling for singing are sung, and the most simple form, in which nothing is sung. For the choice of parts to be sung, those should be first that of their nature are more important and particularly those sung by the priest and congregation together. Later other parts, for the congregation alone or the choir alone, may be added gradually.”
“16. A liturgical celebration can have no more solemn or pleasing feature than the whole assembly’s expressing it faith and devotion in song. Thus an active participation that is manifested by singing should be carefully fostered along these lines:
a. It should include especially the acclamations, responses to the greetings of the priest and the ministers and responses in litanies, the antiphons and psalms, the verses of the responsorial psalm, and other similar verses, hymns, and canticles.
b. Pertinent catechesis as well as actual practice should lead the people gradually to a more extensive and indeed complete participation in all the parts proper to them.
c. Some of the congregational parts may be assigned to the choir alone, however, especially when the people are not yet sufficiently trained or melodies for part singing are used. But the people are not to be excluded from the other parts proper to them. The practice of assigning the singing of the entire Proper and Ordinary of the Mass to the choir alone without the rest of the congregation is not to be permitted.”
"III SINGING DURING MASS
27. As far as possible, eucharistic celebrations with the people, especially on Sundays, should by preference take the form of a Mass with singing, even more than once in the same day.
The distinction between the solemn, the high, and the low Mass, sanctioned by the 1958 Instruction (no. 3) remains in force, according to tradition and current law. But for pastoral reasons degrees of solemnity for the sung Mass are proposed here in order that it will become easier, in accord with each congregation’s capability, to make the celebration of Mass more solemn through the use of singing.
These degrees must be so employed, however, that the first may always be used without the others, but the second and third never without the first. Thus in all cases the faithful are to be brought to take part fully in the singing.
To the first degree belong:
a. in the entrance rites
– the priest’s greeting and the congregation’s response;
– the opening prayer
b. in the liturgy of the word
– the gospel acclamations.
c. in the liturgy of the eucharist
– the prayer over the gifts;
– the preface, with the opening dialogue and the Sanctus;
– the Lord’s Prayer, with the invitation and embolism;
– the greeting May the peace of the Lord;
– the prayer after communion;
– the final dismissal.
To the second degree belong:
a. Kryrie, Gloria, Agnus Dei;
b. profession of faith;
c. general intercessions.
To the third degree belong:
a. songs for the entrance procession and for communion;
b. chants after a lesson or epistle;
c. Alleluia before the gospel;
d. songs for the presentation of the gifts;
e. the Scripture readings, except when it seems better not to have them sung."