From Musicam Sacram
emphasis added
15. The faithful fulfil their liturgical role by making that full, conscious and active participation which is demanded by the nature of the Liturgy itself and which is, by reason of baptism, the right and duty of the Christian people.13 This participation
(a) Should be above all internal, in the sense that by it the faithful join their mind to what they pronounce or hear, and cooperate with heavenly grace,14
(b) Must be, on the other hand, external also, that is, such as to show the internal participation by gestures and bodily attitudes, by the acclamations, responses and singing.15
The faithful should also be taught to unite themselves interiorly to what the ministers or choir sing, so that by listening to them they may raise their minds to God.