Whisk together the soy sauce, brown sugar, sherry, mustard, and garlic powder in a bowl. Arrange the chicken wings in a 9x13-inch baking dish, and pour marinade over the chicken. Cover the baking dish with plastic wrap, and marinate in the refrigerator for at least 2 hours.
Preheat an oven to 375 degrees F (190 degrees C). Remove the plastic wrap from the baking dish.
Bake in the preheated oven until the chicken wings are no longer pink in the center, about 1 hour, basting occasionally.