Pius X, XI, and XII on congregational singing at Mass

Special efforts are to be made to restore the use of the Gregorian Chant by the people, so that the faithful may again take a more active part in the ecclesiastical offices, as was the case in ancient times. (Tra le sollecitudini, Pius X, 1903)

In order that the faithful may more actively participate in divine worship, let them be made once more to sing the Gregorian Chant, so far as it belongs to them to take part in it. It is most important that when the faithful assist at the sacred ceremonies, or when pious sodalities take part with the clergy in a procession, they should not be merely detached and silent spectators, but, filled with a deep sense of the beauty of the Liturgy, they should sing alternately with the clergy or the choir, as it is prescribed. If this is done, then it will no longer happen that the people either make no answer at all to the public prayers -- whether in the language of the Liturgy or in the vernacular -- or at best utter the responses in a low and subdued manner. (Divini Cultus, Pius XI, 1928)

Therefore, they are to be praised who, with the idea of getting the Christian people to take part more easily and more fruitfully in the Mass, strive to make them familiar with the "Roman Missal," so that the faithful, united with the priest, may pray together in the very words and sentiments of the Church. They also are to be commended who strive to make the liturgy even in an external way a sacred act in which all who are present may share. This can be done [...] in high Masses when they answer the prayers of the minister of Jesus Christ and also sing the liturgical chant. (Mediator Dei, Pius XII, 1947)

It is the duty of all those to whom Christ the Lord has entrusted the task of guarding and dispensing the Church's riches to preserve this precious treasure of Gregorian chant diligently and to impart it generously to the Christian people. [...] May it thus come about that the Christian people begin even on this earth to sing that song of praise it will sing forever in heaven. (Musicae Sacrae, Pius XII, 1955)

In solemn Mass there are three degrees of the participation of the faithful: a) First, the congregation can sing the liturgical responses. These are: Amen; Et cum spiritu tuo; Gloria tibi, Domine; Habemus ad Dominum; Dignum et justum est; Sed libera nos a malo; Deo gratias. Every effort must be made that the faithful of the entire world learn to sing these responses. b) Secondly, the congregation can sing the parts of the Ordinary of the Mass: Kyrie, eleison; Gloria in excelsis Deo; Credo; Sanctus-Benedictus; Agnus Dei. Every effort must be made that the faithful learn to sing these parts, particularly according to the simpler Gregorian melodies. But if they are unable to sing all these parts, there is no reason why they cannot sing the easier ones: Kyrie, eleison; Sanctus-Benedictus; Agnus Dei; the choir, then, can sing the Gloria, and Credo. In connection with this, the following Gregorian melodies, because of their simplicity, should be learned by the faithful throughout the world: the Kyrie, eleison; Sanctus-Benedictus; Agnus Dei of Mass XVI from the Roman Gradual; the Gloria in excelsis Deo, and Ite, missa est-Deo gratias of Mass XV; and either Credo I or Credo III. In this way it will be possible to achieve that most highly desirable goal of having the Christian faithful throughout the world manifest their common faith by active participation in the holy Sacrifice of the Mass, and by common and joyful song. c) Thirdly, if those present are well trained in Gregorian chant, they can sing the parts of the Proper of the Mass. This form of participation should be carried out particularly in religious congregations and seminaries. (De Musica Sacra, Sacred Congregation for Rites (during Pius XII), 1958)