Under-Chapter I: General Concepts

Is this enumerated definition-
3. There are two kinds of Masses: the sung Mass ("Missa in cantu"), and the read Mass ("Missa lecta"), commonly called low Mass.
There are two kinds of sung Mass: one called a solemn Mass if it is celebrated with the assistance of other ministers, a deacon and a sub-deacon; the other called a high Mass if there is only the priest celebrant who sings all the parts proper to the sacred ministers.
Under- Chapter II: General Norms
Is this instruction:
14. a) In sung Masses only Latin is to be used. This applies not only to the celebrant, and his ministers, but also to the choir or congregation. (Please note that this particular instruction is listed as subsection “a” of #14. “a” continues discussing when and how vernacular hymns may be appropriately and expressly used. Item “b” has a similar instruction.
c) It is strictly forbidden for the faithful in unison or for a commentator to recite aloud with the priest the parts of the Proper, Ordinary, and canon of the Mass. This prohibition extends to both Latin, and a vernacular word-for-word translation. Exceptions will be enumerated in paragraph 31. However, it is desirable that a lector read the Epistle and Gospel in the vernacular for the benefit of the faithful at low Masses on Sundays and feast days. Between the Consecration, and the Pater noster a holy silence is fitting.

It occurs to me that there are at least two ways to account for this apparent inconsistency;
1. A less-than satisfactory translation of the Latin that, in English, is rendered “to recite aloud.”
2. That specific phrase is to be taken literally, almost as a redundant precaution to insure the integrity of the “Missa in cantu.”

I think that item #21 settles the inconsistency as follows:

The Sacred Text
21. Everything which the liturgical books prescribe to be sung, either by the priest and his ministers, or by the choir or congregation, forms an integral part of the sacred liturgy.

In addition, under
Chapter III-1. Principal liturgical functions in which sacred music is used.

Is this instruction, paying particular heed to item “b”:

A. Mass
a. General principles regarding the participation of the faithful:
22. By its very nature, the Mass requires that all present take part in it, each having a particular function.
a) Interior participation is the most important; this consists in paying devout attention, and in lifting up the heart to God in prayer. In this way the faithful "are intimately joined with their High Priest...and together with Him, and through Him offer (the Sacrifice), making themselves one with Him" (Mediator Dei, Nov. 20, 1947: AAS 39 [1947] 552).
b) The participation of the congregation becomes more complete, however, when, in addition to this interior disposition, exterior participation is manifested by external acts, such as bodily position (kneeling, standing, sitting), ceremonial signs, and especially responses, prayers, and singing.
The Supreme Pontiff Pius XII, in his encyclical on the sacred liturgy, Mediator Dei, recommended this form of participation:
"Those who are working for the exterior participation of the congregation in the sacred ceremonies are to be warmly commended. This can be accomplished in more than one way. The congregation may answer the words of the priest, as prescribed by the rubrics, or sing hymns appropriate to the different parts of the Mass, or do both. Also, at solemn ceremonies, they may alternate in singing the liturgical chant (AAS 39 [1947] 560)".
When the papal documents treat of "active participation" they are speaking of this general participation (Mediator Dei: AAS 39 [1947] 530-537), of which the outstanding example is the priest, and his ministers who serve at the altar with the proper interior dispositions, and carefully observe the rubrics, and ceremonies.

David, it seems that this exhortation, occurring before #31 (“More Perfect Worship) with its degrees seems to abet an ideal of maximal congregational participation as it is licitly outlined. I think there’s more evidence for that judgment than that which you rightly point out might appear to be self-contradiction.

Ok, I think I have an answer to my question. The document above is 1958, so it doesn't apply to the New Mass. So the above document applies to the Traditional Mass, Sung and Recited, and the New Mass form should follow Vatcan II documents and GIRM. (I hope I'm in the right direction, although I'm still confused about the 4 sandwich Mass with sung Ordinary parts that we have in many local parishes.)
(Again, this is very hard for average musicians to comprehend all. I think many musicians don't bother to read, because they get more confused. And you think you got it, then there are more documents to read. It seems endless. If you don't read them, you feel like an ignorant musician, not that I remember them all even after the reading. I think I spend more time reading than actual singing and practicing, and I'm still confused with so many things. Hopefully this is just me. I wish there is one clear instruction for each form, so musicians don't have to read the endless amount of documents. I feel a bit frustrated. I hope some people have good advice on this so we can help others that they don't feel overwhelmed and frstrated with reading documents.)

Mia's post made me think, not for the first time, that it ought to be enshrined in Canon Law that the Church or Pope or a Vatican congregation is not to issue an encyclical, instruction, motu proprio, chirograph, etc, with an incipit identical to that of a previous encyclical, instruction, motu proprio, chirograph, etc, except for a change of suffix indicating case or declension.

"it seems that many of our clerical leaders are, uh, concerned with other ecclesial agendae."
Amen.
And many of them, when informed by someone who has gone to the trouble of educating themselves, uh, wish that that someone would shut the heck up.

(Save the Liturgy, Save the World)

Someone might ask though How about the "pastoral decision" that Vatican II gave to local Bishops and pastors? Are the lay people allow to question their competency? Whether I think praise band worship is right or not, if the pastor think this is bringing more youth to the church, which I don't agree, I don't know whether I can find a document to show it is strcktly forbidden after 1965. If there is one, not a collection of documents that have to be interpretedand and cause an unending dispute, I'd like to know. This would be a big help to many parishes.
Vatican II gave options which are almost boundless. Almost anything can be used as steps to whatever the pastor think is the goal for each parish. Bring as many people as you can by giving the styles of music they like and feel comfortable. People will develope Catholic faith throuhgh other means than music in due time. This is what I get being in average 21st Catholic church in US, and Vatican II seems to support or allow at least this kind of ideas.
I know I may sound a bit ignorant, but these are the questions many of the average Catholics can ask, if they are even interested in doing and singing the right music.

http://www.adoremus.org/1958Intro-sac-mus.html

Introduction
In our time the Supreme Pontiffs have issued three important documents on the subject of sacred music: the Motu proprio Inter sollicitudines of Saint Pius X, Nov. 22, 1903; the Apostolic constitution Divini cultus of Pius XI of happy memory, Dec. 20, 1928; and the encyclical Musicæ sacræ disciplina of the happily reigning Supreme Pontiff Pius XII, Dec. 25, 1955. Other papal documents have also been issued, along with decrees of the Sacred Congregation of Rites in regard to sacred music.

As I re-read this document, only now does that second sentence of the document strike me as strange.
Two paragraphs later is mentioned Mediator Dei.
Okay, thanks, that is ONE other papal document.
There must be at least one more, to be able to use correctly the plural "documents".

So, forum readers ...
Please name another papal document, "in our time", 1903 to 1958, on "sacred music",
not already listed in this post.

Not only has it not been derogated, De musica sacra was cited in the footnotes of Musicam sacram (1967).

I am noticing errors in the English translation of this document.

Here it is in Latin.

The proper title is "Instructio de Musica Sacra et Sacra Liturgia". The "Instructio" part seems to always be left out because it was in a bigger font size. Leaving it out would translate to "on Sacred Music and Sacred Liturgy" which doesn't make as much sense. Even a quick Google search shows how often it is left out.

Chapter III is actually titled "Normae speciales", Special Norms, but for some reason it gets put under Chapter III part 1 to look like an additional subcategory, and looks like this:

Special Norms (no. 11-21).

This is wrong because "Special Norms" is Chapter III (no. 22-118).

One last thing I've noticed is that the title of Chapter III, part 1, section A, subsection e (no. 38-39) is formatted improperly within the body of the text. It looks like a continuation of no. 37. But if you look more closely at the Latin version, you'll notice that the title is indented a little bit more with a centered alignment, and is in italics - just like the other subsections for section A. Whoever put this into English made a mistake in thinking that it was meant to go under no. 37 since it coincidently ends with a subsection d. Subsection e is really the title for no.38-39.

not everything is fit to cross the threshold, insofar as music for use in the liturgy is concerned.

I think almost every 20th century "CDCSM" says this in different ways.