Preheat the oven to 350 degrees F. Place the uncooked rice in the bottom of a 9x13 baking dish. Evenly sprinkle half of the onion soup mix over the rice. Place the chicken pieces on top of the rice. Evenly distribute the mushrooms around the chicken. In a bowl or large measuring cup, whisk together the cream of chicken soup and milk until blended. Pour evenly over the chicken. Sprinkle the remaining onion soup mix over everything. Cover the pan with aluminum foil. Place in the oven and bake at 350 degrees F for 1 1/2 hours or until the chicken is cooked through.