46 We are not unaware that, for serious reasons, some quite definite exceptions have been conceded by the Apostolic See. We do not want these exceptions extended or propagated more widely, nor do We wish to have them transferred to other places without due permission of the Holy See. Furthermore, even where it is licit to use these exemptions, local Ordinaries and the other pastors should take great care that the faithful from their earliest years should learn at least the easier and more frequently used Gregorian melodies, and should know how to employ them in the sacred liturgical rites, so that in this way also the unity and the universality of the Church may shine forth more powerfully every day.
Source: Musicae Sacrae (Vatican.va)