Why was voice over used in the movie midway for the japanese admiral?

Yes, Paul Frees.

Frees was often called upon in the 1950s and 1960s to "re-loop" the dialogue of other actors, often to correct for foreign accents, lack of English proficiency, or poor line readings by non-professionals. These dubs extended from a few lines to entire roles. This can be noticed rather clearly in the film Midwaywhere Frees reads for Toshirō Mifune's performance as Admiral Yamamoto.