The “anthrax in the envelope” was paragraph 32 of the 1967 Instruction. It says:
In some places there exists the lawful practice, occasionally confirmed by indult, of substituting other songs for the Introit, Offertory and Communion chants in the Graduale Romanum. At the discretion of the competent territorial authority this practice may be retained, on condition that the songs substituted fit in with those parts of the Mass, the feast, or the liturgical season. The texts of such songs must also have the approval of the same territorial authority. (DOL 4153)
[…] The fact is that this rule of unlimited substitution has practically swept away the Proper of the Mass. Moreover, it also effectively removed the norms that the Council had established for liturgical music in general. In recent times, not a single territorial authority in the world has interfered in what is sung at Mass – save that sometimes they protested against the use of traditional ecclesiastical chant… If any such territorial authority actually tried to intervene, there would be no canons to which they could appeal. Had there been any such canon, they would have possessed mere legal power but no actual competence entitling them to take a stand on questions such as: what is secular?, what is worthy of the liturgy?, what is and what is not in keeping with the parts of the Mass? And finally, if they actually did take a stand, no one took notice of it. The fact is that in the universal Church today, it is only the caprice of the local priest, cantor or lay committee of the parish council (each changing from time to time, from person to person, from place to place) which determines what will be sung as the Introit of, for example, the Sixteenth Sunday of Ordinary Time.
Experience reminds us that there is still another consequence. At the very moment when the choice of chants is left in the hands of local personnel instead of the Church Universal, the standard of measurement changes from objective norms to so-called “pastoral needs,” which is but a euphemism for the real or imaginary taste of those present at the liturgy. Thus all demands upon, and norms governing, musica sacra become illusory. No song can be rejected because it is unworthy of the liturgy, for the counter-argument is always at hand: “Our people like it”; “This congregation favors it”; “The song is fitting for this age group,” and so on and on.
Laszlo Dobszay, The Bugnini-Liturgy and the Reform of the Reform (2003)