The mission the monkeys are found on is named "Rebirth". The monkeys are found once Hudson starts talking to you. You will hear loud screeching and find monkeys in cages on the walls stacked on top of one another. Before you enter the big circular room, you will see a corner that is full of cages on your right. Directly before this hallway on the right side, is a AK-74u with a grenade launcher attachment (the room has pigs inside it). Grab the AK-74u and equip the grenade launcher attachment with . Back away from where the monkeys are at and aim so that the grenade will hit the floor. This should kill all the monkeys in the corner.