In a large skillet, heat butter over medium-high heat; saute mushrooms until tender, 4-5 minutes. Transfer to a 4-qt. slow cooker. Stir in broth, cream of chicken soup and soup mix. Stir in meatballs. Cook, covered, on low until flavors are blended, 5-6 hours., Stir in sour cream and parsley. Cook, covered, until heated through, about 15 minutes. If desired, serve with noodles or mashed potatoes.