BROWN meat with onions and peppers in large skillet on medium-high heat. Stir in chili powder; cook 1 min. Add salsa and corn; mix well. Simmer 5 min.
SPREAD 1 cup meat sauce onto bottom of 8- or 9-inch square baking dish; top with layers of 1 tortilla, 1/2 cup meat sauce, sour cream and 1/4 cup cheese. Cover with 1 tortilla, 1 cup of remaining meat sauce and 1/4 cup of remaining cheese; top with remaining tortilla and meat sauce. Cover with foil.
BAKE 25 min. or until casserole is heated through. Top with remaining cheese; bake, uncovered, 5 min. or melted.