So, this is a tough subject for many English-speaking singers: how to pronounce Latin. Often, conductors speak of Latin as if there is only one kind of Latin. They say, “Remember that in Latin, …” or “Well, in Latin, you don’t …”
It’s important to remember that as soon as the Romance languages began springing off into their own, there was no longer a standard way of pronouncing it. Over the centuries, classical scholars were able to reconstruct the way Latin sounded in ancient times by examining contemporary accounts and poetic forms. This kind of Latin (“Classical Latin”) still survives in Latin courses across the word. Its v sounds like a w. It has more than five vowels. There is no word stress, but only syllable length. This kind of Latin rarely is seen in sung situations, other than some college songs (Gaudeamus igitur for instance) and pieces that were written at universities that stressed the classical restored pronunciations (such as the three Latin motets of Charles Stanford).
It wasn’t until 1903 that the Pope at the time, Pope Pius X, issued a decree (a motu proprio, as it is called) indicating indirectly that the expectation for singing in the Roman Catholic Church was in the way people pronounced Latin in Rome, Italy, which is done by reading the Latin as if it is Italian. This became what is now known in the United States as “Church Latin” or “Ecclesiastical Latin.” Even after this decree was made, many countries were slow to adopt Italian Latin as a standard pronunciation, particularly countries without a strong Roman Catholic presence.
To determine which Latin is correct, one must first look at the composer’s nationality, then the composer’s time. If the piece is written before 1903, one pronounces Latin in the working language of the composer. After 1903, it is a judgment call. Germany rejected Italian Latin from the start and maintained a strong German Latin tradition (based on a scientific standardization that they undertook, which is slightly different from the German Latin heard by Mozart and Beethoven). England seems to have used English Latin or Classical Latin until around the 1920s. France used French Latin until around the 1930s, when it really started catching on, but with still some French accents.
I’ve assembled a few resources here to help people with their questions on Latins that match the style of the pieces that they are singing. Most of my information comes from Harold Copeman’s 1990 book Singing in Latin and Timothy McGee’s Singing Early Music.