Hey, It's That Voice!: "Dark Avenger" and "Defender" are narrated by Orson Welles. In Battle Hymns MMXI, "Dark Avenger" is narrated by Christopher Lee. Yes, that one. If it means anything, Christopher Lee has expressed a liking for Manowar. That ought to tell you enough, really.
And for Kings of Metal MMXIV, the narrator for "The Warrior's Prayer" is none other than BRIAN BLESSED, replacing the late Arthur "Pendragon" Wilshire.