We were on the topic of liturgical music before as well as reverence at Mass. Here are a few excepts from the papal encyclical defining and explaining sacred music:
"Nothing should have place, therefore, in the temple calculated to disturb or even merely to diminish the piety and devotion of the faithful, nothing that may give reasonable cause for disgust or scandal, nothing, above all, which directly offends the decorum and sanctity of the sacred functions and is thus unworthy of the House of Prayer and of the Majesty of God. We do not touch separately on the abuses in this matter which may arise. Today Our attention is directed to one of the most common of them, one of the most difficult to eradicate, and the existence of which is sometimes to be deplored in places where everything else is deserving of the highest praise – the beauty and sumptuousness of the temple, the splendor and the accurate performance of the ceremonies, the attendance of the clergy, the gravity and piety of the officiating ministers. Such is the abuse affecting sacred chant and music. …
And it is vain to hope that the blessing of heaven will descend abundantly upon us, when our homage to the Most High, instead of ascending in the odor of sweetness, puts into the hand of the Lord the scourges wherewith of old the Divine Redeemer drove the unworthy profaners from the Temple. …
- Sacred music should consequently possess, in the highest degree, the qualities proper to the liturgy, and in particular sanctity and goodness of form, which will spontaneously produce the final quality of universality. It must be holy, and must, therefore, exclude all profanity not only in itself, but in the manner in which it is presented by those who execute it. …
But it must, at the same time, be universal in the sense that while every nation is permitted to admit into its ecclesiastical compositions those special forms which may be said to constitute its native music, still these forms must be subordinated in such a manner to the general characteristics of sacred music that nobody of
any nation may receive an impression other than good on hearing them.
- These qualities are to be found, in the highest degree, in Gregorian Chant, which is, consequently the Chant proper to the Roman Church, the only chant she has inherited from the ancient fathers, which she has jealously guarded for centuries in her liturgical codices, which she directly proposes to the faithful as her own…
On these grounds Gregorian Chant has always been regarded as the suprememodel for sacred music, so that it is fully legitimate to lay down the following rule: the more closely a composition for church approaches in its movement, inspiration and savor the Gregorian form, the more sacred and liturgical it becomes; and the more out of harmony it is with that supreme model, the less worthy it is of the temple.
Pope Piux X, 1903, Tra le Sollecitudini
adoremus.org/TraLeSollecitudini.html#anchor40134803