Little known fact about voice actors: they have to go into a sound studio and actually record lines of dialogue for these videogames. For many years, the understanding was that developers would steal the souls of voice talent and use mystical coding techniques to get their voices into the games. This is not the case.
This 2015 D.I.C.E. Summit talk explains the entire history of Call of Duty's zombie mode
10:30 AM on 02.12.2015
Call of Duty unleashes DLC, zombie horde on PlayStation and PC in late February
5:00 PM on 02.04.2015