191 As regards music, let the clear and guiding norms of the Apostolic See be scrupulously observed. Gregorian chant, which the Roman Church considers her own as handed down from antiquity and kept under her close tutelage, is proposed to the faithful as belonging to them also. In certain parts of the liturgy the Church definitely prescribes it; it makes the celebration of the sacred mysteries not only more dignified and solemn but helps very much to increase the faith and devotion of the congregation. For this reason, Our predecessors of immortal memory, Pius X and Pius XI, decree - and We are happy to confirm with Our authority the norms laid down by them - that in seminaries and religious institutes, Gregorian chant be diligently and zealously promoted, and moreover that the old Scholae Cantorum be restored, at least in the principal churches. This has already been done with happy results in not a few places.
Source: Mediator Dei (Vatican.va)