Things piled up, and this post is out of date, but I wanted to link to this excellent
post at Rorate Coeli by Carlos Antonio Palad (incidentally, there is a prayer request for one of the contributors at that excellent blog, to which I gladly acquiesce.)
At my usual level of informedness I thought, when I dragged the long suffering Himself into Chicago, that it was a High Mass. He is indifferent enough to the EF without my subjecting him to a music-less liturgy, it seems… he rightly discerned, (do I wince loudly….?,) that yes, I was a bit disappointed that he will not make a bit of an effort to follow in a missal, he says he is happy just to let the “sacramenty goodness wash over” him.
I think he misunderstands my level of knowledge -- I am only now after maybe nine or ten times really getting the hang of it. And I did learn, thanks to last Wednesday’s Mass that it is actually easier to follow without the distraction, (sorry!,) of gorgeous music.
I think the only Low Mass I’d been to before (that I remember, of course as a child I was probably dragged on occasion, although both of my parents being singers, they were not too frequent,) had badly sung, in some cases very corny, hymns and motets.
I was also unaware (I really must be getting oblivious,) that in the old calendar it was NOT the feast of Pope St Gregory, but it was the equally appropriate day to rejoice in the anniversary of such a great document, the feast of St Pius X, (whom as a child I assumed was the editor of the bigger and better hymnal of my dad’s -- I knew the guy the smaller blue one was named for was only gone…)

50 years of De Musica Sacra

On September 3, 1958, the Feast of Pope St. Pius X, the Sacred Congregation of Rites released the Instruction De musica sacra et sacra liturgia (Instruction on Sacred Music and the Sacred Liturgy).
This instruction crowned the great series of papal documents on sacred music promulgated beginning in 1903: the Motu proprio Inter sollicitudines of St. Pius X (Nov. 22, 1903); the Apostolic Constitution Divini cultus of Pius XI (Dec. 20, 1928); and the encyclical Musicæ sacræ disciplina of the Ven. Pius XII (Dec. 25, 1955). It also embodies much of the teaching of Mediator Dei (November 20, 1947), Pope Pius XII's great encyclical on the Sacred Liturgy.
It remains until today the guiding document on sacred music for communities and the faithful attached to the Gregorian Rite. Furthermore, all Catholics -- even those who serve or worship exclusively in the context of the Novus Ordo -- would do well to study this document and think with it, given that it embodies the general principles of sacred music as set forth by the Magisterium.

As the introduction to the instruction itself states:

Before his encyclical on sacred music, the Supreme Pontiff, Pius XII, issued another encyclical on the sacred liturgy, Mediator Dei (November 20, 1947), which very clearly explains, and demonstrates the relation between liturgical doctrine, and pastoral needs. It has therefore been considered appropriate to put together from the above documents a special instruction containing all the main points on sacred liturgy, sacred music, and the pastoral advantages of both. In this way their directives may be more easily, and securely put into practice.

As the Gregorian Rite reclaims its rightful place in the heart of the Church, the instruction deserves to be studied thoroughly once again, and most importantly, obeyed.
Some quotes (emphases mine):

5. Gregorian chant, which is used in liturgical ceremonies, is the sacred music proper to the Roman Church; it is to be found in the liturgical books approved by the Holy See. This music has been reverently, and faithfully fostered, and developed from most ancient, and venerable traditions; and even in recent times new chants have been composed in the style of this tradition.
This style of music has no need of organ or other instrumental accompaniment.

6. Sacred polyphony is measured music which arose from the tradition of Gregorian chant. It is choral music written in many voice-parts, and sung without instrumental accompaniment. It began to flourish in the Latin Church in the Middle Ages, and reached its height in the art of Giovanni Pierluigi Palestrina (1524-1594) in the latter half of the sixteenth century; distinguished musicians of our time still cultivate this art.

7. Modern sacred music is likewise sung in many voice-parts, but at times with instrumental accompaniment. Its composition is of more recent date, and in a more advanced style, developed from the previous centuries. When this music is composed specifically for liturgical use it must be animated by a spirit of devotion, and piety; only on this condition can it be admitted as suitable accompaniment for these services.

8. Sacred music for organ is music composed for the organ alone. Ever since the pipe organ came into use this music has been widely cultivated by famous masters of the art. If such music complies with the laws for sacred music, it is an important contribution to the beauty of the sacred liturgy.
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10. Religious music is any music which, either by the intention of the composer or by the subject or purpose of the composition, serves to arouse devotion, and religious sentiments. Such music" is an effective aid to religion" (Musicæ sacræ disciplina, idem.). But since it was not intended for divine worship, and was composed in a free style, it is not to be used during liturgical ceremonies.
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16. Gregorian chant is the music characteristic of the Roman Church. Therefore, its use is not only permitted, but encouraged at all liturgical ceremonies above all other styles of music, unless circumstances demand otherwise...
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17. When the choir is capable of singing it, sacred polyphony may be used in all liturgical ceremonies. This type of sacred music is specially appropriate for ceremonies celebrated with greater splendor, and solemnity.
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24. The more noble form of the Eucharistic celebration is the solemn Mass because in it the solemnities of ceremonies, ministers, and sacred music all combine to express the magnificence of the divine mysteries, and to impress upon the minds of the faithful the devotion with which they should contemplate them. Therefore, we must strive that the faithful have the respect due to this form of worship by properly participating in it in the ways described below.

25. In solemn Mass there are three degrees of the participation of the faithful:
a) First, the congregation can sing the liturgical responses. These are: Amen; Et cum spiritu tuo; Gloria tibi, Domine; Habemus ad Dominum; Dignum et justum est; Sed libera nos a malo; Deo gratias. Every effort must be made that the faithful of the entire world learn to sing these responses.

b) Secondly, the congregation can sing the parts of the Ordinary of the Mass: Kyrie, eleison; Gloria in excelsis Deo; Credo; Sanctus-Benedictus; Agnus Dei. Every effort must be made that the faithful learn to sing these parts, particularly according to the simpler Gregorian melodies. But if they are unable to sing all these parts, there is no reason why they cannot sing the easier ones: Kyrie, eleison; Sanctus-Benedictus; Agnus Dei; the choir, then, can sing the Gloria, and Credo.
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29. The first way the faithful can participate in the low Mass is for each one, on his own initiative, to pay devout attention to the more important parts of the Mass (interior participation), or by following the approved customs in various localities (exterior participation)
Those who use a small missal, suitable to their own understanding, and pray with priest in the very words of the Church, are worthy of special praise. But all are not equally capable of correctly understanding the rites, and liturgical formulas; nor does everyone possess the same spiritual needs; nor do these needs remain constant in the same individual. Therefore, these people may find a more suitable or easier method of participation in the Mass when "they meditate devoutly on the mysteries of Jesus Christ, or perform other devotional exercises, and offer prayers which, though different in form from those of the sacred rites, are in essential harmony with them" (Mediator Dei, AAS 39 [1947] 560-561).
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45. Where the ancient, and venerable custom of singing Vespers according to the rubrics together with the people on Sundays, and feast days is still practiced, it should be continued; where this is not done, it should be re-introduced, as far as possible, at least several times a year.
The local Ordinary should take care that the celebration of evening Masses does not interfere with the practice of singing Vespers on Sundays, and feast days...
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48. Compositions of sacred polyphony, by the old masters as well as by contemporary artists, are not to be introduced into the liturgy unless it has first been established that, either in their original form or in arrangements, they comply fully with the ideals, and admonitions set forth in the encyclical Musicæ sacræ disciplina (AAS 48 [1956] 18-20). If there is any doubt, the diocesan commission [ha ha!] on sacred music is to be consulted.
49. Ancient manuscripts of this music still lying about in archives should be uncovered, and if necessary, steps taken for their preservation. Musicologists should make critical editions of them as well as editions suitable for liturgical use.
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97. Those who have a part in the sacred music -- composers, organists, choir directors, singers, and instrumentalists -- should above all be outstanding Christians, and give example to the rest of the faithful, conformable to their role as direct or indirect participants in the sacred liturgy.

98. Besides excelling in Christian faith and morals, these persons must also possess the training necessary to fulfill their particular role of participation in the liturgy.

a) Composers of sacred music should have a thorough knowledge of the historical, dogmatic or doctrinal, practical, and rubrical aspects of the liturgy; they should know Latin; [okay, may I just say....? I don't know WHAT to say! If only, huh?] and finally they should be well trained in the art, and the history of both sacred, and secular music.

b) Organists, and choir directors should also have a comprehensive knowledge of the liturgy, and a sufficient understanding of Latin; and finally they should be well trained in their art, and able to carry out their role worthily, and competently.

c) Singers, both boys and adults, should be taught the meaning of the liturgical functions, and of the texts they sing insofar as they are capable of comprehending, for then their singing will be inspired by an understanding mind, and a loving heart, and be truly rendered as befits the service of an intelligent person. They should also be taught to pronounce the Latin words correctly, and distinctly. Pastors, and those directly in charge must see to it that good order, and true devotion reign in that part of the church occupied by the singers.

d) Instrumentalists who perform sacred music should not only be well trained in the techniques of their instruments, but should also know how to adapt them to the playing of sacred music. They should be well enough instructed in the sacred liturgy that their devotion will be evidenced by an artistic performance.