NO SOUND

Hello,
I HAVE EXTERNAL SPEAKERS ON MY PC, I CAN SEE THAT THE MUSIC IS PLAYING ON, LIKE MYSPACE, BUT I CANT HEAR IT. THE LITTLE SPEAKER ICON IN THE LOWER RIGHT HAND CORNER ISN'T THERE AND I DON'T KNOW IF ITS ON MUTE THROUGH MY KEYBOARD. I HAVE TURNED THE VOLUME WAY UP BUT STILL CANT HEAR ANYTHING. I REMOVED MY SPEAKERS AND HOOKED THEM TO MY STEREO HEADPHONE JACK AND THEY WORK FINE, WHAT COULD BE THE PROBLEM?