Many Catholic choirmasters do not realize there are two ways to write hymns in Latin: (1) quality (which usually does not rhyme); (2) stress-accent (which usually rhymes). Rhythm by quality refers to the pattern of long and short vowels in Latin, and is sometimes referred to as a more “noble” or “aristocratic” way to construct a hymn. Rhythm by stress-accent ignores the long and short qualities and is only interested in the tonic accent, and is sometimes called a more “vulgar” form of poetry. For whatever reason, over a period of centuries, rhythm by stress-accent became the most common way to write Latin hymns. Fr. Matthew Britt (The Hymns of the Breviary & Missal, Pages 26-27) has this to say (emphasis mine):
The Romans learned their poetry, as they learned the other fine arts, from the Greeks . . . This poetry, it need scarcely be said, was strictly quantitative. But together with this classical poetry there co-existed, and that too from the beginning of Latin letters, a purely rhythmical poetry, a poetry of the people, in which the ballads and folk songs of the common people were written. The common people knew nothing of quantity with its artificial and arbitrary rules which the poets had made. Quantitative poetry was therefore the poetry of the educated; rhythmical or accentual poetry was the poetry of the common people. Now, the early hymns of the Church were likewise the songs of the people, and were necessarily written in a manner that would appeal to all the people and not merely to the cultured classes. This was effected by St. Ambrose and by the earlier writers of the Ambrosian school, by a compromise between the quantitative and the rhythmical principles. These writers made use of the simplest of all the lyric meters, the Iambic Dimeter, with its regular succession of short and long syllables; but they took care that the accents should in general fall on the long syllables. Their quantitative hymns can therefore be read rhythmically. In the composition of his hymns, St. Ambrose did not make use of any greater licenses than did Horace and his contemporaries. Later on, however, it is noticeable that less and less attention was paid to quantity and greater attention to accent which began to replace it. As early as the fifth century many hymn-writers employed the rhythmical principle only. This process continued until in the Middle Ages all sense of long and short syllables had vanished, and hymns were written in accentual, non-quantitative meters. In studying the hymns chronologically, it will be observed also that the growth of rhyme kept pace with the growth of accent.
During my entire life, I have only met three (3) priests who observed long and short syllables while speaking Latin. If the reader is curious about this, he can download this Guide to Pronunciation (PDF), but I don’t recommend doing so, since so few priests pronounce long and short syllables.
Here is way most Catholic priests pronounce Latin:
Guide No. 1 (PDF) — Excerpted from Parish Book of Chant (CMAA, 2008)
Guide No. 2 (PDF) — Excerpted from Mass & Vespers (Solesmes, 1957)
Guide No. 3 (PDF) — Excerpted from ‘Proper’ of the Mass (Carlo Rossini, 1933)
Guide No. 4 (PDF) — Excerpted from the Liber Usualis (Solesmes, 1961)
Guide No. 5 (PDF) — Excerpted from A Textbook of Gregorian Chant (Dom Gregory Suñol, 1929)
Guide No. 6 (PDF) — Excerpted from Basic Gregorian Chant (Sister Demetria, 1960)
Guide No. 7 (PDF) — Excerpted from Chants of the Church (Solesmes, 1953)
Guide No. 8 (PDF) — Excerpted from Gregorian Chants for Church and School (Goodchild, 1944)
Guide No. 9 (PDF) — Excerpted from A New School of Gregorian Chant (Johner, 1925)
Guide No. 10 (PDF) — Excerpted from Fundamentals of Gregorian chant (Heckenlively, 1950)
47-Page Book (PDF) — Correct Pronunciation of Latin According to Roman Usage (De Angelis, 1937)
The Parish Book of Chant is probably the best. In particular, pay careful attention to the “common pitfalls” Americans fall into, like saying “inn” instead of “een” for the Latin word in. Or saying, “ih-mack-yoo-lah-tuh” instead of “ee-mah-coo-lah-tah” for the Latin word immaculáta. Or saying “Doe-mee-nay” instead of “Doh-mee-neh” for the Latin word Dómine.
That being said, these rules can be taken too far. I’ve served the Latin Mass for priests from all over the world: Australia, Austria, Germany, France, England, America, Mexico, Puerto Rico etc. However, it is rare to hear a priest say “cheh-lee” for the Latin word cæli. Most say “chay-lee,” and there’s no use losing sleep over this!