As Musicam Sacram draws to complete its first chapter outline of general norms, we read of matters of repertoire and style.
First we see a pragmatic statement which pretty much opens up the repertoire to any suitable genre:
9. In selecting the kind of sacred music to be used, whether it be for the choir or for the people, the capacities of those who are to sing the music must be taken into account. No kind of sacred music is prohibited from liturgical actions by the Church as long as it corresponds to the spirit of the liturgical celebration itself and the nature of its individual parts,[SC 116] and does not hinder the active participation of the people.[SC 28]
The guiding principles are first, liturgical suitability, then active participation.
10. In order that the faithful may actively participate more willingly and with greater benefit, it is fitting that the format of the celebration and the degree of participation in it should be varied as much as possible, according to the solemnity of the day and the nature of the congregation present.
Section ten reminds us of a very important principle: good liturgy is not achieved by a sameness in format or participation level. The clergy, people, and musicians do not, for themselves in any way, have 100% maximum participation as a goal. Liturgical factors as well as pastoral considerations will contribute to the judgment about what gets sung, and by whom. Variety in form and theme is essential to any good work of art.
11. It should be borne in mind that the true solemnity of liturgical worship depends less on a more ornate form of singing and a more magnificent ceremonial than on its worthy and religious celebration, which takes into account the integrity of the liturgical celebration itself, and the performance of each of its parts according to their own particular nature. To have a more ornate form of singing and a more magnificent ceremonial is at times desirable when there are the resources available to carry them out properly; on the other hand it would be contrary to the true solemnity of the liturgy if this were to lead to a part of the action being omitted, changed, or improperly performed.
This section provides the strongest argument for a sober approach when great musical resources are at hand. If the rubrics state, for example, that the Sanctus is sung by the priest with the people, a choral performance is clearly an “improper” performance from a liturgical view. MS demonstrates, at least to me, how and why traditional five-movement sung Masses were judged by most Catholic musicians to be of questionable applicability–even from 1967 on. One need not look beyond MS to find the justification.
12. It is for the Holy See alone to determine the more important general principles which are, as it were, the basis of sacred music, according to the norms handed down, but especially according to the Constitution on the Liturgy. Direction in this matter, within the limits laid down, also belongs to the competent territorial Episcopal Conferences of various kinds, which have been legitimately constituted, and to the individual bishop.[SC 22]
We conclude Chapter I with the Consilium’s statement that Rome is responsible for laying down the “more important general principles,” something we see in the GIRM. From there, bishops’ conferences, then in turn, individual bishops will fill in the gaps (or not, as the case may be) from there.
It becomes clear that authentic legislation on church music will only be complete by a confluence of guidance on norms and the completion of the liturgical reform from Rome, as well as particulars added by bishops on the national and diocesan level. I don’t think the Consilium ever envisioned a complete body of music legislation originating with the pope or the curia. The expectation that this isntruction provided it or began its provisionwould seem to go against the explicit text in section 12.