As far as Gregorian chant and sacred polyphony being sung in the context of the Sacred Liturgy, there ought to be no other options of pronunciation other than to use ecclesiastical (church) Latin. Classical Latin just won't do the pieces justice. The reason is pretty simple and straightforward actually. The Italians are the modern day descendants of the Romans. With Rome being the headquarters of the Roman Catholic Church, the Italian way of pronunciation ought to be used in the Roman Rite everywhere. In 1903, Pope St. Pius X decreed Italian pronunciation to be the norm in liturgical settings, so if any of you wants to pick a fight over this, go ask Pope St. Pius X himself! :P
Some recurring items to point out:
- The letter 'J' should always be pronounced like the letter 'Y', e.g. judicare (yoo-dee-ka-reh), jejunio (yeh-yoo-nee-oh).
- The letter 'H' is ALWAYS SILENT except for cases such as mihi (meek-kee) and nihil (neek-il). Thus hominibus is oh-mee-nee-boos while hosanna is oh-san-na NOT hoh-san-na.
- Virgine is pronounced as veer-jee-nay. Note the veer does not sound like 'vir' virtue but like the 'd' in 'deer' replaced by a 'v'.
- Excelsis is pronounced as ex - chair-el - sis.
- Amen is always a-men NOT aye-man.
- Ecclesia is eck-clair-see-ah NOT eh-curl-see-ah.
- Words with -gn, e.g. magnus is pronounced as mah-nyoos, agnus pronounced as ah-nyoos. Subsequently magnificat is pronounced as mah-nyee-fee-cart.