Fr. Kirby cited this in the context of discussing the preference for propers over hymns. Even more strongly, he said that it is not permitted that hymns replace propers. I've never heard of such a clear statement.
I went looking for the text of this judgement. Then I came to this piece by Susan Benofy, who cites the document:
Abandoning the traditional music and texts of the Mass was clearly not the intention of the Council, whose Constitution on the Sacred Liturgy, Sacrosanctum Concilium (1963), decreed that "the treasury of sacred music is to be preserved and fostered with great care" (SC 114). This principle was further clarified in 1969 by the Consilium (the group of bishops and experts set up by Pope Paul VI to implement the Constitution on the Liturgy), who responded to an inquiry on whether the permission for singing vernacular hymns at a low Mass — given in the instruction De musica sacra et sacra liturgia of September 3, 1958 — was still in effect. (Before the Council the hymns sung at low Mass did not replace the prescribed Mass texts, but were an addition to them, and were considered only an "indirect" form of participation.)
The Consilium's response was very clear:
That rule [permitting vernacular hymns] has been superseded. What must be sung is the Mass, its Ordinary and Proper, not "something", no matter how consistent, that is imposed on the Mass. Because the liturgical service is one, it has only one countenance, one motif, one voice, the voice of the Church. To continue to replace the texts of the Mass being celebrated with motets that are reverent and devout, yet out of keeping with the Mass of the day amounts to continuing an unacceptable ambiguity: it is to cheat the people. Liturgical song involves not mere melody, but words, text, thought and the sentiments that the poetry and music contain. Thus texts must be those of the Mass, not others, and singing means singing the Mass not just singing during Mass.
What this means is striking to say the least. The idea here is that the postconcilar period has removed permission for hymns to be sung at Low Mass and replaced it with a positive obligation to sing the propers in the postconciliar Mass.
Amazed? I am. Truly. The implications are rather astounding actually.
Even more remarkably, the document was cited in 1993 by the US Bishops Committee on the Liturgy: BCL Newsletter, Volume XXIX, August-Sept 1993, paragraph 9-11.
In spite of these efforts to promote the sung liturgy, preference continues to be given to singing during the Mass instead of singing the Mass. In fact many of the faithful interpret singing the liturgy to mean singing hymns or songs. Thus those involved in liturgical preparations oftentimes confine themselves to the selection of hymns as their first priority and neglect the singing of ritual texts. Likewise many composers give preference to the composition of hymns and other sacred songs rather than to the ritual texts of the liturgy....
In 1969 the Sacred Congregation of Rites responded to a [sic] inquiry on whether the instruction of September 3, 1958, which allowed for singing of four vernacular hymns during a recited Mass still applied. The response, published in Notitiae, stated: "That rule [permitting vernacular hymns] has been superseded. What must be sung is the Mass, its Ordinary and Proper, not "something", no matter how consistent, that is imposed on the Mass. Because the liturgical service is one, it has only one countenance, one motif, one voice, the voice of the Church. To continue to replace the texts of the Mass being celebrated with motets that are reverent and devout, yet out of keeping with the Mass of the day amounts to continuing an unacceptable ambiguity: it is to cheat the people. Liturgical song involves not mere melody, but words, text, thought and the sentiments that the poetry and music contain. Thus texts must be those of the Mass, not others, and singing means singing the Mass not just singing during Mass." (Notitiae 5 [1969] 406).
Still trying to think through the implications here...
