105 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 more than one way, when, for instance, the whole congregation, in accordance with the rules of the liturgy, either answer the priest in an orderly and fitting manner, or sing hymns suitable to the different parts of the Mass, or do both, or finally in high Masses when they answer the prayers of the minister of Jesus Christ and also sing the liturgical chant.
Source: Mediator Dei (Vatican.va)