192 Besides, "so that the faithful take a more active part in divine worship, let Gregorian chant be restored to popular use in the parts proper to the people. Indeed it is very necessary that the faithful attend the sacred ceremonies not as if they were outsiders or mute onlookers, but let them fully appreciate the beauty of the liturgy and take part in the sacred ceremonies, alternating their voices with the priest and the choir, according to the prescribed norms. If, please God, this is done, it will not happen that the congregation hardly ever or only in a low murmur answer the prayers in Latin or in the vernacular." A congregation that is devoutly present at the sacrifice, in which our Savior together with His children redeemed with His sacred blood sings the nuptial hymn of His immense love, cannot keep silent, for "song befits the lover" and, as the ancient saying has it, "he who sings well prays twice." Thus the Church militant, faithful as well as clergy, joins in the hymns of the Church triumphant and with the choirs of angels, and, all together, sing a wondrous and eternal hymn of praise to the most Holy Trinity in keeping with words of the preface, "with whom our voices, too, thou wouldst bid to be admitted."
Source: Mediator Dei (Vatican.va)