Preheat oven to 350°F (325°F for dark or nonstick pan). Prepare cake batter as instructed on the box for a 13x9-inch pan. Let the cake cool for 10 minutes, then gently run a knife around the edges of the pan to release the cake. Transfer the cake to a cooling rack and let it cool completely, approximately 1 hour. For easier handling, chill the cake in the refrigerator or freezer for 30 to 60 minutes until firm.
Cut the cake using a serrated knife. Refer to the link for a visual guide. For the piano bench, slice a 4 1/2x1 1/2-inch piece and then cut it horizontally to create a smaller bench if preferred. Transfer the cake onto a tray. Fill a small resealable food-storage plastic bag with about 2 tablespoons of frosting. Apply a thin layer of frosting on the cake to seal in the crumbs. Chill in the refrigerator or freezer for 30 to 60 minutes to allow the frosting to set. Proceed to frost the entire cake.
Cut 1 bar of white chocolate into 1 3/4x1/2-inch strips. Use the remaining bar to cut a 2 1/2x1 1/2-inch piece for the musical component. Proceed to slice the milk chocolate into 1x3/8-inch pieces. Arrange the white and milk chocolate pieces on the cake to resemble piano keys. Carefully snip a small corner of the frosting bag and pipe musical notes onto the white chocolate "music." Position the music above the keys and store the cake loosely covered.