This toy concept is based on magnetic repulsion and attraction. On the inside face of each disc is a distinct spatial arrangement/ratio of magnets, so that each disc produces a different rate of rotation. The main disc is operated by a hand crank, which induces the rotation of each successive disc. Located on the circumference of each disc is a mallet that hits a corresponding tone-producing key upon each complete rotation. These keys can be interchanged to produce different melodies or sounds.
Although this product would appeal to a broad audience, children would especially be intrigued by the properties of magnetism and the notion that an invisible force "magically" governs our everyday surroundings.
I imagine that the majority of the toy's components to be made out of some variety of hardwood and metal for the axis and keys. Considering the construction and parts would be fairly simple, the cost probably wouldn't exceed $40.00, the magnets contributing most to the total cost. As far as safety concerns, the magnets might pose a choking hazard for young children. To avoid this issue, the toy would be marketed for 5+.