Chapter II-B. Different Elements of the Mass
Reading and Explaining the Word of God
9. When the Scriptures are read in the Church, God Himself is speaking to His people, and Christ, present in His own word, is proclaiming the Gospel.
The readings must therefore be listened to by all with reverence; they make up a principal element of the liturgy. In biblical readings God's word addresses all people of every era and is understandable to them, but a living commentary on the word, that is, the homily, as an integral part of the liturgy, increases the word's effectiveness. 
Prayers and Other Parts Assigned to the Priest
10. Among the parts assigned to the priest, the eucharistic prayer is preeminent; it is the high point of the entire celebration. Next are the prayers: the opening prayer or collect, the prayer over the gifts, and the prayer after communion. The priest, presiding over the assembly in the person of Christ, addresses these prayers to God in the name of the entire holy people and all present.  Thus there is good reason to call them "the presidential prayers."
11. It is also up to the priest in the exercise of his office of presiding over the assembly to pronounce the instructions and words of introduction and conclusion that are provided in the rites themselves. By their very nature these introductions do not need to be expressed verbatim in the form in which they are given in the Missal; at least in certain cases it will be advisable to adapt them somewhat to the concrete situation of the community.  It also belongs to the priest presiding to proclaim the word of God and to give the final blessing. He may give the faithful a very brief introduction to the Mass of the day (before the celebration begins), to the Liturgy of the Word (before the readings), and to the eucharistic prayer (before the preface); he may also make comments concluding the entire sacred service before the dismissal.
12. The nature of the presidential prayers demands that they be spoken in a loud and clear voice and that everyone present listen with attention.  While the priest is reciting them there should be no other prayer and the organ or other instruments should not be played.
13. But the priest does not only pray in the name of the whole community as its president; he also prays at times in his own name that he may exercise his ministry with attention and devotion. Such prayers are said inaudibly.
Other Texts in the Celebration
14. Since by nature the celebration of Mass has the character of being the act of a community,  both the dialogues between celebrant and congregation and the acclamations take on special value;  they are not simply outward signs of the community's celebration, but the means of greater communion between priest and people.
15. The acclamations and the responses to the priest's greeting and prayers create a degree of the active participation that the gathered faithful must contribute in every form of the Mass, in order to express clearly and to to further the entire community's involvement. 
16. There are other parts, extremely useful for expressing and encouraging the people's active participation, that are assigned to the whole congregation: the penitential rite, the profession of faith, the general intercessions, and the Lord's Prayer.
17. Finally, of the other texts:
a)Some constitute an independent rite or act, such as the Gloria, the responsorial psalm, the Alleluia verse and the verse before the gospel, the Sanctus, the memorial acclamation, and the song after communion.
b)Others accompany another rite, such as the songs at the entrance, at the preparation of the gifts, at the breaking of the bread (Agnus Dei), and at communion.
Vocal Expression of the Different Texts
18. In texts that are to be delivered in a clear, loud voice, whether by the priest or by the ministers or by all, the tone of voice should correspond to a genre of the text, that is, accordingly as it is a reading, a prayer, an instruction, an acclamation, or a song; the tone should also be suited to the form of celebration and to the solemnity of the gathering. Other criteria are the idiom of different languages and the genius of peoples.
In the rubrics and in the norms that follow, the words say (dicere) or proclaim (proferre) are to be understood of both singing and speaking, and in accordance with the principles just stated.
Importance of Singing
19. The faithful who gather together to await the Lord's coming are instructed by the Apostle Paul to sing psalms, hymns, and inspired songs (see Col 3:16). Song is the sign of the heart's joy (see Acts 2:46). Thus St. Augustine says rightly: "To sing belongs to lovers."  There is also the ancient proverb: "One who sings well prays twice."
With due consideration for the culture and ability of each congregation, great importance should be attached to the use of singing at Mass; but it is not always necessary to sing all the texts that are of themselves meant to be sung.
In choosing the parts actually to be sung, however, pEndnotes should be given to those that are more significant and especially to those to to be sung by the priest or ministers with the congregation responding or by the priest and people together. 
Since the faithful from different countries come together ever more frequently, it is desirable that they know how to sing at least some parts of the Ordinary of the Mass in Latin, especially the profession of faith and the Lord's Prayer, set to simple melodies. 
Movement and Posture
20. The uniformity in standing, kneeling, or sitting to be observed by all taking part is a sign of the community and the unity of the assembly; it both expresses and fosters the spiritual attitude of those taking part. 
21. For the sake of uniformity in movement and posture, the people should follow the directions given during the celebration by the deacon, the priest, or another minister. Unless other provision is made, at every Mass the people should stand from the beginning of the entrance song or when the priest enters until the end of the opening prayer or collect; for the singing of the Alleluia before the gospel; while the gospel is proclaimed; during the profession of faith and the general intercessions; from the prayer over the gifts to the end of the Mass, except at the places indicated later in this paragraph. They should sit during the readings before the gospel and during the responsorial psalm, for the homily and the presentation of the gifts, and, if this seems helpful, during the period of silence after communion. They should kneel at the consecration unless prevented by the lack of space, the number of people present, or some other good reason.
But it is up to the conference of bishops to adapt the actions and postures described in the Order of the Roman Mass to the customs of the people.  But the conference must make sure that such adaptations correspond to the meaning and character of each part of the celebration.
22. Included among the external actions of the Mass are those of the priest going to the altar, of the faithful presenting the gifts, and their coming forward to receive communion. While the songs proper to these movements are being sung, they should be carried out becomingly in keeping with the norms prescribed for each.
23. Silence should be observed at the designated times as part of the celebration.  Its function depends on the time it occurs in each part of the celebration. Thus at the penitential rite and again after the invitation to pray, all recollect themselves; at the conclusion of a reading or the homily, all meditate briefly on what has been heard; after communion, all praise God in silent prayer.