I have done a little bit of modding in my time Slyspy but I have never meddled with voices. I think I may have found the necessary files though.

programfiles/sega/medievalIItotalwar/data/sounds

They should be evident when on this page but if not I believe they are:

voice1.dat
voice1.idx
voice2.dat
voice2.idx
voice3.dat
voice3.idx

Maybe try the modding forum? It should be simple enough.