First I will try to explain a little about magnetism and sound.
 As you may already know magnets have north and south poles like the earth does. Think of the north pole as having a positive charge and the south negative. You may also know the unlike magnetic fields attract and like fields repel. We use this phenomena to our advantage to produce sound.
 Sound waves are just that, waves of air moving at different frequencies. You can look at it much like a wave of water, how it has a up and a down cycle, so does sound. When a speaker moves out it produces the up cycle of the wave. When it moves in it produces the down cycle. But how?

  When a positive voltage is applied to the + side of the voice coil the speaker will move outward. To see this get a 1.5 volt AAA, AA, C, OR D battery and connect the positive  battery terminal to + speaker etc. You will see the speaker move outward. Switch them to see it move inward. 

 When a voltage is applied to the coil it produces a magnetic field of its own. If that field is opposite of the magnet it will pull the cone down. if it is the same it will repel the cone away or out. The higher the voltage, the more the cone will move. If you turn it up to far the cone can get thown to hard up and lock up never to move again or the coil can over heat.

 Sound is measured in decibels and cycles per second. Humans can hear from 20 cycles per second to 20,000 cycles per second. We say this 20 hertz to 20 kilohertz.
 one hertz is the speaker moving out and in once.
 So a 20hz bass drop will move the speaker in and out 20 times per second. It takes a big speaker to produce 20 hz and a small one for 20khz. This is why woofers are big and tweeters are small.