Dell inspiron 1440 sound problem

Greetings!
Could anyone please help me? I think there is a problem on my laptop, dell inspiron 1440 (windows 7, Core 2 duo). because when I plug in earphones, the sound should only come from the earphones right? but it does not happen that way. what happens is, the laptop speakers and the earphones release sound at the same time. how can make the sound come from the earphones only and not come from both the speakers and the earphones? Many thanks!