Jackie and Faldage are both correct about pronunciation in singing. And it isn't just a matter of how you pronounce Latin etc. Even when singing in English there are times when the normal spoken pronunciation of a vowel needs to be altered so that the mouth is better shaped to enunciate the musical tone. You will have noticed that singers generally have their mouths open and rounded, which is to produce a sonorous tone; but you can't pronounce the short flat vowel sounds with your mouth like that, so for euphony the vowel is made deeper or more round. On the other hand, especially in Latin but sometimes in other languages, a final 'e' sound has to be shortened to 'eh' for the sake of the musical tone. It is also necessary frequently to suppress the letter 's' so a choir doesn't sound like the reptile house at the zoo.
