Musicam Sacram sets down some more liturgy basics, and we begin to get into principles which we can apply to music.
6. The proper arrangement of a liturgical celebration requires the due assignment and performance of certain functions, by which “each person, minister or lay(person), should carry out all and only those parts which pertain to (their) office by the nature of the rite and the norms of the liturgy.”[SC 28] This also demands that the meaning and proper nature of each part and of each song be carefully observed. To attain this, those parts especially should be sung which by their very nature require to be sung, using the kind and form of music which is proper to their character.
I don’t see any reason to interpret “kind and form” as liturgical. The Creed and the Lord’s Prayer, though inspirational for composers to set to music, are simply not by nature sung pieces of music. MS also speaks of musical form. Here I would suggest they are speaking of natural forms the various parts of the Mass take: litanies, hymns, and acclamations, as well as song forms of antiphon-plus-psalmody.
7. Between the solemn, fuller form of liturgical celebration, in which everything that demands singing is in fact sung, and the simplest form, in which singing is not used, there can be various degrees according to the greater or lesser place allotted to singing. However, in selecting the parts which are to be sung, one should start with those that are by their nature of greater importance, and especially those which are to be sung by the priest or by the ministers, with the people replying, or those which are to be sung by the priest and people together. The other parts may be gradually added according as they are proper to the people alone or to the choir alone.
The Consilium begins to break down the stark separation between the High Mass and Low Mass, giving a practical direction to effectively put every celebrated Mass on target toward an ideal of “fuller form.”
MS lays out a plan: emphasize parts which are of greater importance, especially those involving liturgical dialogue (including litanies and psalms) and those which are sung by the entire assembly (acclamations and hymns). “Choir alone” music is relegated to the least priority.
8. Whenever, for a liturgical service which is to be celebrated in sung form, one can make a choice between various people, it is desirable that those who are known to be more proficient in singing be given preference; this is especially the case in more solemn liturgical celebrations and in those which either require more difficult singing, or are transmitted by radio or television.[Instruction of the S.C.R., 3 September 1958, n. 95.]
There’s not a sense here that singing proficiency is limited to the relative judgment of lay people or clergy separate from each other. The sense here is that is a lay person is permitted a sung role, and can perform that role better than a priest, the lay person should be selected.
If, however, a choice of this kind cannot be made, and the priest or minister does not possess a voice suitable for the proper execution of the singing, he can render without singing one or more of the more difficult parts which concern him, reciting them in a loud and distinct voice. However, this must not be done merely for the convenience of the priest or minister.
No musical slacking, in other words.