Mass doesn’t
have to be sung, but it is strongly encouraged. As for rules about singing the Mass, the 1967 Instruction
Musicam Sacram says the following:These degrees are so arranged that the first may be used even by itself, but the second and third, wholly or partially, may never be used without the first. In this way the faithful will be continually led toward an ever greater participation in the singing.
The following belong to the
first degree:
Code:
(a) In the entrance rites: the greeting of the priest together with the reply of the people; the prayer.
(b) In the Liturgy of the Word: the acclamations at the Gospel.
(c) In the Eucharistic Liturgy: the prayer over the offerings; the preface with its dialogue and the Sanctus; the final doxology of the Canon, the Lord's Prayer with its introduction and embolism; the Pax Domini; the prayer after the Communion; the formulas of dismissal.
The following belong to the
second degree:
Code:
(a) the Kyrie, Gloria and Agnus Dei;
(b) the Creed;
(c) the prayer of the faithful.
The following belong to the
third degree:
Code:
(a) the songs at the Entrance and Communion processions;
(b) the songs after the Lesson or Epistle;
(c) the Alleluia before the Gospel;
(d) the song at the Offertory;
(e) the readings of Sacred Scripture, unless it seems more suitable to proclaim them without singing.So technically, if *anything* is sung during Mass, it should be those elements of the "first degree". This isn't often the case, though. :(