Summary of differences between the modern Italianate pronunciation of church Latin as described in the Liber Usualis (and my Fundamentals of Gregorian Chant according to the Solesmes method) and what appears to be the historical early medieval Roman pronunciation based on the evidence furnished by variant spellings:

C or CC before e and i: ts

SC before e and i: unvoiced s

XC before e or i: ks

Summary of differences from the Germanic pronunciation of church Latin:

Generally, there is no distinction between long and short vowels in sung Latin.

EU: the u in pronounced only in passing to the next syllable; this diphthong does not rhyme with Eng. oy!

G before e and i: as in Eng. generous

GN: as in monsignor or ni in Eng. onion

H: silent except in the words mihi and nihil, where it is like k in accordance with the alternative -ch- spelling of these words

NGU followed by another vowel: the gu is normally like Eng. gw (e.g. lingua, languerunt)

OE: same as ae

QU followed by another vowel: like Eng. kw

S: unvoiced s except between vowels of the same word, then voiced z

SC before e and i: unvoiced s

T: dental; close to d

TI followed by another vowel in the same word: tsi, regardless of the word accent, except after s, t, or x

XC before e or i: ks

Y: same as i

Z: dz

Possible historically justified regional variants:

CC before e and i: kts

G before e and i: like Eng. y

GN: both letters receive their normal pronunciation

H in the words mihi and nihil: aspirate h, as in Eng.

S: voiced also in eleison as Latinization of a Greek word?

Z: as in Eng. (not dz or ts)