[Windows Support] windows asus laptop plays sound on speaker and headphones :S

hey, i have a really annoying problem with my windows laptop. every time i play something at school on my headset the sound hears on both the headset and the laptop speaker. ive been using this laptop for about a year now and this just happened out of the blue :S
since this can be a really annoying problem not just for other ppl at school but also for me, could you please come with a quick solution for this?

thanks in advace,

the icelander :)