Moonlight on the Magic Flute by Mary Pope Osborne

Jack and Annie travel to 18th-century Austria, where they are assigned to help a young genius—Wolfgang Amadeus Mozart. Disguised as servants at a palace party, they must complete a secret mission involving a magical flute and Mozart’s hidden gifts. As they navigate royal drama and mischievous monkeys, they help inspire the young composer. The plot combines historical detail with humor and light fantasy. It's a whimsical, music-filled adventure with a touch of magic.