A theremin is a unique musical instrument with two antennas where one antenna controls pitch and the other controls volume. By waving a hand near the antennas, you can change the pitch and volume without physically touching the device.
So when a cat got near a theremin, it moved around the antenna, creating music. Of course this confused the cat, which caused it to move and make even more music. If you want to know how musically inclined your cat may be, let it near a theremin.