Hat tip to Bear-i-tone.
OK, this may date back to 1933, but a lot (of not all) of this holds true still today. This was written by Fr. Carlo Rossini, then-Organist/Choirmaster at St. Paul's Cathedral in Pittsburgh. Some may recognize Fr. Rossini's name with the Liturgical Organist multi-volume series of short preludes, postludes, and such. I have his first volume, by a very happy accident.
But anyhoo, here's the Decalogue (emphasis mine):
1. Since music in church becomes part of the liturgy it is in itself Worship. Church music, therefore, must be offered in the best possible way. (Does Haugen/Haas/St. Louis Jesuits ditties constitute the "best possible way"? Doubtful.)
2. The first requirement for a good rendition of church music is that the the singers be permeated with the Spirit which prompts Holy Mother Church during the different Seasons of the Ecclesiastical Year (Advent, Christmas, Lent, Easter and Pentecost). (Of course, in 1933 there was no such thing as Ordinary Time. We could easily make an adaptation for that by letting the readings of the day from Holy Scripture set the tone.)
3. Choir members, in order to sing with the proper expression and to give God intelligent praise, should become familiar with the meaning of the phrases of the liturgical text through the English translation of the Missal.
4. Church singing must retain the character of choral music. Consequently, no individual voice in the choir should be heard above the others; no personal ambition or desire to "show off" should be tolerated. (Ah yes, this reminds me of two people - a cantor from an outside parish that a former pastor of mine once hired for Saturday Masses who acted like a diva and left disappointed because she didn't get to do her brand of music - DE-DA-DEE! - and Thomas Day's depicting of "Mr. Caruso" in his book Why Catholics Can't Sing.)
5. The singing should always proceed smoothly(legato). The tedious defect of producing a separate impulse of the voice for each note and syllable should be carefully avoided. The contrary defect of "sliding", "slurring", and "scooping" should also be avoided. (This holds especially true for chant and polyphony. Not always the case for traditional metrical hymnody.)
6. In chanting the Proper of the Mass the tempo to be observed is that of a solemn, oratorical declamation of the text. A short pause will be made after the middle cadence of the psalm -tone and whenever the breathing mark () is found. The recitation on the "long note" (Dominant) of the psalm tone must be done evenly as if each syllable of the text carried an eighth-note.
7. The correct accentuation of the words is an important factor in good singing. In fact, talking and singing follow similar rules, the accented syllable always receiving the greater emphasis.
8. The Italian pronunciation of the Latin is prescribed by the Church. Singers, therefore, should carefully enunciate both vowels and consonants according to the Italian system. (I may post these rules, from 1933 later.) (Is Latin even taught these days anymore? My niece goes to a CATHOLIC high school that doesn't even offer Latin. In fact, the ONLY foreign language they offer is Spanish. I took two years of it in a PUBLIC high school, albeit Classical Latin, over 25 years ago. Even with that, Ecclesiastical Latin isn't hard to adapt. Some pronuciations are different, but the translation is the same.)
9. The organist should never forget that the organ accompaniment must serve only to "support" the singing, and should never overpower the voices. (It should also excite. Not show off, but excite. Let the hymn itself decide registration. Use that registration. Don't drown everyone out with it, if you can help it, though. Most hymns do well with 8'4'2' registration, perhaps a good four-rank mixture, and finish off the last verse with a nice, yet blending, chorus reed.)
10. Liturgical chant in early Christian centuries belonged exclusively to the Choir of Levites, and our church singers today, although laymen, are taking their place. Members of church choirs, therefore, should consider their office as a "privilege" and should show themselves worthy of the same by a dignified, modest and devout bearing. (With this last rule, I direct the readers back to rules 1 and 4.)