Heat oven to 350F. Mix mushroom soup and milk; reserve 1/2 of the cup of the soup mixture. Mix the remaining soup mixture with rice, mushrooms (including liquid), and half of the onion soup mix. Pour into oblong baking dish. Place chicken breasts on top. Pour reserved soup mixture over chicken. Sprinkle with remaining onion soup mix. Cover and bake 1 hour. Uncover and bake 15 minutes longer.