Heat oven to 375°F. Sprinkle both sides of chicken breasts with taco seasoning mix; place in ungreased 8-inch square (2-quart) glass baking dish. Pour salsa over chicken.
Bake 25 to 35 minutes or until chicken is fork-tender and juices run clear. Sprinkle cheese evenly over chicken; bake 3 to 5 minutes longer or until cheese is melted. Serve with sour cream.