When singing Italian music of the 17th century that sets Latin texts, we use an Italian Latin pronunciation. If "projicias" and "sacrificium" were Italian words, then we would not pronounce the "i" in the final syllable. (This is because in Italian pronunciation, the "i" after "c" or "g" softens that consonant when it occurs immediately before "a", "o" or "u".) Yet most choirs seem to pronounce the "i" separately in these two words, making "projicias" a 4-syllable word and "sacrificium" a 5-syllable word. Which would be correct in this context – to pronounce the "i" in the final syllable, or not?
In practice, Italian pronunciation is not as standardized as one might think (both the pronunciation of the Italian language, and the Italian pronunciation of words of the Latin language). So I have some doubts about whether there is a truly definitive answer to questions like "what is correct?"
This issue is discussed on the Italian Wikipedia, which says the pronunciation of -ci- in this context would be either /t͡ʃi/ or /t͡ʃj/, not merely /t͡ʃ/:
c e g in origine indicavano sempre rispettivamente i suoni /k/ e /ɡ/, poi nel latino ecclesiastico andarono ad indicare non solo /k/ e /ɡ/, ma anche rispettivamente /t͡ʃ/ e /d͡ʒ/ davanti alle lettere e ed i (pronunciata sempre, anche se consonantizzata: dulcia si legge /ˈdulkia/ nella pronuncia restituta e /ˈdult͡ʃja/ o /ˈdult͡ʃia/ - ma non /ˈdult͡ʃa/ - nella pronuncia ecclesiastica), oltre che davanti ai dittonghi oe ed ae;
When dealing with musical pieces, you might be able to get a sense of the original intended pronunciation by looking at how many notes the word is set to.