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.
Amen
For those not completely in the know, "Musicam sacram" is a document issued in 1967 by the Congregation for Divine Worship (CDW) as part of the continuing work of Vatican II.
Father Jeff is correct, though it looks like CDW added some interpretation of their own to what was approved by the bishops at the Council! The curia do say, however, that everyone is to look to the Constitution on the Sacred Liturgy for norms. (paragraph 12). Sort of a typical example, the emphases in the Constitution and this document aren't the same, thus setting us all up to pick the documents that make the most sense to us.
It's a point of laughter for me that Musicam Sacram has the nerve to dutifully footnote the Constitution (the little 14 hanging there in the paragraph 15 number a) Father Jeff quotes). However, if one follows the trail to the Constitution paragraph 11, it says merely that our minds should be attuned with our voices, not that participation is primarily interior.
Constitutions from Councils trump curial documents, but I concede that Father Jeff has found a text on which to stand that is not just his own preference or opinion.
Thus the joys of working with the documents of the church!
Not only does Sacrosanctum Concilium mention participation in section 14, but at least another ten times it clarifies in detail why participation is important, what the council bishops thought it should include, and so forth.
Since MS treats sacred music, and we can conclude that participation involves more than music, SC would still trump the 1967 instruction.
As Pope Pius taught in Mediator Dei, the participation of the people in the oblation is not based on the fact that they perform a visible liturgical rite. It is based on their interior worship:
But the conclusion that the people offer the sacrifice with the priest himself is not based on the fact that, being members of the Church no less than the priest himself, they perform a visible liturgical rite; for this is the privilege only of the minister who has been divinely appointed to this office: rather it is based on the fact that the people unite their hearts in praise, impetration, expiation and thanksgiving with prayers or intention of the priest, even of the High Priest himself, so that in the one and same offering of the victim and according to a visible sacerdotal rite, they may be presented to God the Father. It is obviously necessary that the external sacrificial rite should, of its very nature, signify the internal worship of the heart.
(MD, para. 93).
OK, so the Constitution from the Council talks about participation, and Musicam sacram is focused on music only...So the inference is that there are spoken parts for the assembly but the choir sings all the important stuff and it's OK that we listen?
If Father Jeff sings everything from the sign of the cross to the dismissal, there's still a concern with accessibility of the music to the entire assembly!
RC, that's not exactly the emphasis of the council bishops in Sacrosanctum concilium. The interior life is praised, but the council, and the rubrics of the Roman Missal indicate an outward expression in speech, song, gestures, and other outward signs is part of the Roman Rite.