How come sound only comes from one ear?

How come sound only comes from one ear?
When using the headset, sound only comes out of one of the ears. I have tried to mess with the volume control, but no luck.

ANSWERS:

Make sure that the headphone is fully plugged into the audio jack, you'll hear a light click if it's properly done. If you think the speaker cone needs to be replaced, check out the speaker cone replacement guide.