Netbook Play Sounds on Both Headset and Speakers

Hello, I want to ask some questions. I have a problem with my netbook sounds. My netbook is Eee PC ASUS 1001p. Anytime I plugged headset to the port, usually the pop-up windows would appear in screen and ask whether I want to play sound on headset or speaker. But recently, the Pop-up windows not showed, it just automatically play sounds on both speakers and headset. How I could make those pop-up windows appears again so I can choose the output of the sounds, cause it's kinda disturbing me. I appreciate if you may help me. Thank you very much.