Play the Nobody Rides For Free mission, where you need to collect the three boxes of land mines. Play for awhile and a fourth box will appear where you started the level. When you collect it, you will get a message that you have found an Easter Egg. The voice that you unlocked is called "Grandpa".
In the Grave Danger mission (where you must blow open the graves with grenades), behind one of the houses is a very small circle-shaped grave. There is steam or smoke coming out of the grave, just like the big ones. Drop a grenade on top of the grave, jump down into the grave, and you will get a message that you have found an Easter Egg. The voice that you unlocked is called "Horror".
SpecializedFuse1.