Authority for Propers Over Hymns

People write me from time to time to ask for the authority behind my continuing claim that hymns should not replace Mass propers. In addition to the General Instruction and the whole of the sacred music tradition, I often cite the following, which I fear gets lost in the shuffle. Bugnini's own Consilium in 1969 offered the following instruction, consistent with the Vatican II emphasis on chant over vernacular hymnody. As printed in 1 Notitiae, 5 (1969), p. 406

That rule [permitting vernacular hymns] has been superseded.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.