-During a campaign level where you have to fix computer monitor, get yourself some room to breathe by killing the enemies, or do the code as it is, although you are very likely to get blown up. Jump on the keys on the keyboard and spell "TEAM17" (no spaces). You will receive a message at the top of the screen stating that you unlocked a hidden Easter Egg. The secret is a new voice option for teams.
-Play the Nobody Rides For Free mission, where you must collect the three boxes of land mines. Play for some time 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".
-During the Grave Danger mission (where you must blow open the graves with grenades), behind one of the houses is a tiny circle-shaped grave. There is steam or smoke coming out of the grave, just like the large 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".