I know there are several different ways to pronounce Latin. I think no one sings classical music using Classical Latin pronunciation in which, for instance, "c" is pronounced as /k/. I believe the reason is that, during the Medieval era, Latin has evolved so that its pronunciation has changed all around Europe.
However, I've noticed that music ensembles that play Early Music based on a historically informed approach tend to use different pronunciations of that used in nowadays Ecclesiastical Latin. For instance, it's quite usual to hear "c" before "e" pronounced as /ks/ in words such as "luceat eis" or "excelsis" instead of the /tʃ/ nowadays Ecclesiastical pronunciation, but I believe (I'm not sure about that: this is one of the reasons for my question) there are Early Music ensembles that use alternative pronunciations based on regional differences. I've also heard the "qui" of "requiem" pronounced in several different ways.
Can anyone give an introductory overview to that phenomenon?