Heat oven to 400°F. In medium saucepan, heat pizza sauce, meatballs and mushrooms to boiling over medium heat. Cover; cook 10 to 15 minutes or until meatballs are thawed and thoroughly heated.
Meanwhile, partially cut buns lengthwise, leaving one side attached. On ungreased cookie sheet or shallow pan, place buns, cut side up. Bake 3 to 5 minutes or until toasted.
Spoon meatball mixture onto 1 side of toasted buns. Sprinkle with cheese. Bake 2 to 4 minutes longer or until cheese is melted.