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.
28. The distinction between 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.
29. 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.
30. To the second degree belong:
a. Kyrie, Gloria, Agnus Dei;
b. profession of faith;
c. general intercessions.
31. 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.
32. In some places there is the lawful practice, occasionally confirmed by indult, of substituting other songs for the entrance, offertory, and communion chants in the Graduale. At the discretion of the competent territorial authority this practice may be kept, on condition that the songs substituted fit in with those parts of the Mass, the feast, or the liturgical season. The texts of such songs must also have the approval of the same territorial authority.
33. The assembly of the faithful should, as far as possible, have a part in singing the Proper of the Mass, especially by use of the simpler responses or other appropriate melodies.
Of all the chants for the Proper the one coming between the readings as a gradual or responsorial psalm is particularly significant. It is intrinsically a part of the liturgy of the word and thus is to be sung with the whole assembly sitting, listening, and even, if possible, taking part.
34. When there is to be part-singing for the chants of the Ordinary of the Mass, they may be sung by the choir alone in the customary way, that is, either a capella or with instrumental accompaniment. The Congregation, however, must not be altogether left out of the singing for the Mass.
In other cases the chants of the Ordinary may be divided between choir and congregation or between one part of the congregation and another. The singing is then done by alternating verses or in any other way that takes in most of the entire text. It is important in any such arrangement, however, to attend to the following. Because it is a profession of faith, the Credo is best sung by all or else sung in a manner that allows the congregation's proper participation. Because it is an acclamation concluding the preface, the Sanctus should as a rule be sung by the entire assembly along with the priest. Because it accompanies the breaking of the bread, the Agnus Dei may be repeated as often as necessary, especially in concelebrations and it is appropriate as well for the congregation to have a part in it, at least by singing the final Grant us peace.
35. The congregation should join the priest in singing the Lord's Prayer.  When it is in Latin, it is sung to the traditional melodies; the melodies for singing it in the vernacular must have the approval of the competent territorial authority.
36. Any one of the parts of the Proper or the Ordinary in a low Mass may be sung. Sometimes it is even quite appropriate to have other songs at the beginning, at the presentation of the gifts, and at the communion, as well as at the end of Mass. It is not enough for these songs to be "eucharistic" in some way; they must be in keeping with the parts of the Mass and with the feast or liturgical season.