Once you’ve decided how you’re going to pronounce Latin when you’re speaking English, the next problem comes up for singers.
It’s not only English that has its own version of Latin pronunciation, but every language in Europe has its idiosyncratic ways as well. In German, for instance, they pronounce C before E and I (and AE and OE) as /ts/, while G is always /g/. Consonantal U~V is /v/, like Italian.
Should English-speaking choirs pronounce the text of masses written by Beethoven or Schubert (the question never seems to come up with Haydn or Mozart) with the Italianate Latin, or a more German-sounding pronunciation? Likewise, should Fauré’s Requiem be sung in the way that French choirs sing it? (French composers often set the words with final stress, as French, rather than keeping to the Latin pattern.) In my experience as the rehearsal accompanist for a Choral Society, it depends on the knowledge of the conductor. Our present one knows how Germans pronounce Latin, so he likes the singers to use /ts/ and /g/, and the other quirks, when they sing Schubert, but he is less sure of how the French pronounce Latin, so for Fauré they sing the Italianate style. However he also likes Italianate Latin for Bruckner. The English composer Douglas Coombes was commissioned to write a Requiem for a French choir. They took so long to rehearse it that our choir gave the first performance, using Italianate pronunciation. The work sounded very different when the French choir came to England to sing it with their French pronunciation.
This conundrum of what to do with Latin could be carried on to other languages: should Brahms’ Ein Deutsches Requiem be sung with a Hamburg accent, while Schubert’s songs have a Viennese accent, and Richard Strauss’s lieder a Bavarian one? Taking it even further, should we sing songs using Burns’ words with a Scottish accent, or in the accent of the composer? I have heard Spanish singers distinguish between Castilian and South American pronunciations within the same recital.
This could all get very silly, but actors reading stories aloud often adopt the appropriate accent.
Getting back to Latin pronunciation, Harold Copeman wrote a comprehensive study (Singing in Latin) of the ways in which Latin is pronounced in the various European languages. This was self-published by Mr Copeman in 1990, together with a separate “Pocket” version of the same work. I don’t know whether it’s still available new, but the only copy I can find on Abebooks is now priced at £225!