If this is from the Red Book / Llibre Vermell of Montserrat, the vernacular language of the book's users would have been Catalan, not Middle English. I think there's a CD by Jordi Savall of material from this manuscript which includes this piece.

A youtube here
gives some melodies for this piece.

Medieval latin pronunciation varied by region, and was certainly allied to regional accent, but it's not quite true to say that it would have been pronounced by the 'rules' of the regional accent--there were no rules yet. And the inhabitants of England would certainly have been able to distinguish sung Latin from sung English!