Coat a large nonstick skillet with cooking spray; add oil. Place over medium-high heat until hot. Add mushrooms and next 3 ingredients; sauté until tender. Drain mixture, and transfer to a bowl. Add beans, olives, and black pepper, stirring well.
Spoon one-fourth of bean mixture down center of each tortilla. Top each with 1 tablespoon sour cream, 1 tablespoon salsa, and 2 tablespoons cheese; fold opposite sides over filling.
Wipe skillet with a paper towel. Coat pan with cooking spray; place over medium-high heat until hot. Place burritos, seam sides down, in pan; cook 1 minute on each side or until lightly browned. Serve with remaining 1/2 cup salsa.