You could add a music box to your greeting card in several ways. One way would be to find a music box that is the same size as a greeting card and attach it to the inside of the card. Another way would be to create a small pocket on the front or back of the card and put the music box in there. Lastly, you could also create a hole in the center of the greeting card and put the music box string through it so that the music plays when you open up the card.