HEAT oven to 375ºF.
BROWN meat with peppers and onions in large skillet on medium-high heat. Stir in chili powder; cook 1 min. Add salsa and corn; mix well. Simmer 5 min.
SPREAD 2 cups meat sauce onto bottom of 13x9-inch baking dish; top with layers of 2 tortillas, 1 cup meat sauce, sour cream and 1/2 cup cheese. Cover with 2 tortillas, 2 cups meat sauce and 1/2 cup cheese; top with remaining tortillas and meat sauce. Cover.
BAKE 30 min. or until casserole is heated through. Top with remaining cheese; bake, uncovered, 5 min. or until melted.