From Musicam Sacram:

"28. The distinction between solemn, sung and read Mass, sanctioned by the Instruction of 1958 (n. 3), is retained, according to the traditional liturgical laws at present in force. However, for the sung Mass (Missa cantata), different degrees of participation are put forward here for reasons of pastoral usefulness, so that it may become easier to make the celebration of Mass more beautiful by singing, according to the capabilities of each congregation.

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.

29. The following belong to the first degree:

(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.

30. The following belong to the second degree:

(a) the Kyrie, Gloria and Agnus Dei;
(b) the Creed;
(c) the prayer of the faithful.

31. The following belong to the third degree:

(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."

"36. There is no reason why some of the Proper or Ordinary should not be sung in said Masses. Moreover, some other song can also, on occasions, be sung at the beginning, at the Offertory, at the Communion and at the end of Mass. It is not sufficient, however, that these songs be merely "Eucharistic" -- they must be in keeping with the parts of the Mass, with the feast, or with the liturgical season."

My questions:

1. WASN'T IT SEVERAL YEARS before the music to, for example, the entrance rites was even made available???

2. Since it says "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" for "the sung Mass (Missa cantata)" and then it says in 36 "There is no reason why some of the Proper or Ordinary should not be sung in said Masses" are there different rules for the sung Mass and said Mass? (just like Gilbert asked)