The concept behind a speaker is electromagnetism. Electricity is run through two coils, placed end-on-end to one another. This spiraling of electric current turns the coils that the current passes through into magnets. These magnets generate an attractive or repulsive force upon each-other depending on the relative directions of the current within them. Now, by rapidly adjusting the current, you can alter the amount of magnetic field, and thus the force, that the two coils exert upon each other. This means that you can rapidly move one coil, the "speaker", in and out of the boom box. As it does so, it displaces the air (if it is being moved out) or creates a vacuum (if it is being drawn in). This movement causes pressure waves - sound - to be formed in the air.