Bryan A. Garner’s Dictionary of Modern Legal Usage has an entertaining article on LAW FRENCH, which concludes with a note on pronunciation:
English and, to a lesser extent, American lawyers have generally preserved the medieval pronunciations given to Law French terms—pronunciations that resemble modern English much more than they do modern French. So the “correct” pronunciation of oyez is /oh-yez/ or /oh-yes/, not /oh-yay/, and of autrefois acquit /oh-tər-foyz/, not /oh-tər-fwah/. *See J.H.Baker, Manual of Law French (2d ed. 1990); J.H.Baker, “Law French,” in 7 Guide to American Law 80-81 (1984).
Heikki E.S. Mattila, Comparative Legal Linguistics, writes
common-law lawyers normally pronounce phrases from law French and Latin in the same way as genuinely English words. For example, when the court begins its sessions the usher cries three times: Oyez! This word of French origin is pronounced: oou-yès, with the accent placed on the second syllable. Satirists have not been slow to comment on this pronunciation: according to one writer, When the Crier cried “Oyez!”, the people cried “O no!” As for Latin, the expression stare decisis, for example, gives a good idea of the pronunciation of English-speaking lawyers. They say (according to Black’s Law Dictionary): stahr-ee or stair-ee di-sI-sis.
[...] Law French comes from the Normandy of the 11th century and was never pronounced in the same way as modern French. [Critics] do not understand young American lawyers, who — according to Peter Tiersma — “are increasingly pronouncing Law Latin in the style of Julius Caesar, and Law French in the style of Brigitte Bardot”.
So Baker and Black would seem to be your best sources.
No doubt Law French and Law Latin had originally whatever pronunciation passed for proper Latin and Norman-French pronunciation in the 12th century, and that has evolved normally through the Great Vowel Shift and other intervening phonological calamities.