Read this, translated by Siliconera:
In an interview with Japanese outlet Automaton, developers at Square Enix and Tose responsible for the recent Crisis Core: FFVII Reunion discussed some details about the development of this remaster. The Crisis Core: FFVII Reunion developers talked about the methods used to remaster a 15-year-old PSP game for modern platforms, the technology used for lip sync, and the process behind adding full voice acting to the game.
Crisis Core: FFVII Reunion uses the CRI LipSync technology from CRI Middleware to emulate realistic lip sync in both the English and Japanese dubs of the game. The game uses a pre-analysis method, which transforms the voice audio into files that determine the movement of the mouth, and can be manually adjusted if needed.
Program Director Haruyuki Yamaguchi of Tose said that adapting the game to Unreal Engine 4 was a relatively easy process. The original PSP game was originally programmed using C++, and since Unreal Engine 4 also uses C++ as its base, there wasn’t much difficulty when changing engines.