The music box reassures Winnie because it is pretty, and no one who owns something like that could really be all that bad.
The music box is one of Mae Tuck’s most prized possessions. She finds in comforting and enjoys listening to it year after year.
Then she stood and took from the washstand beside the bed a little square-shaped object, a music box painted with roses and lilies of the valley. It was the one pretty thing she owned and she never went anywhere without it. (Ch. 2)
The music box is important because it is how the man in the yellow suit connects the spring to the Tucks. She has had the music box for so long that he has heard its music described. When they hear the music, Winnie’s grandma thinks it is elves playing, but Winnie thinks it is a music box. The man in the yellow suit knows it’s not elves. He thinks it is the music of the immortals.
Winnie finds the music box reassuring because it is pretty, and the music coming from it is pretty. It reminds her of home, where she first heard it.
And then, after several more turns, the music began to play again, brisk from its fresh winding, and merry. No one who owned a thing like this could be too disagreeable. Winnie examined the painted roses and lilies of the valley, and smiled in spite of herself. (Ch. 6)
The music box calms all of them. It is soothing music and a distraction from their troubles. Winnie is nervous because she has been “kidnapped” by people she does not know. The Tucks are nervous because they are not sure how to explain everything to Winnie. To most people, their tale of drinking from a spring in the woods and becoming immortal must sound strange.