I've a Dell Optiplex 960 (which I presume has a speaker inside the tower). This speaker is normally in use whenever I play music (lets call this "tower speaker"), and when I insert my headphones in the 3.5mm jack (of the tower), the tower speaker stops and the music is played through the headphones (an obvious thing to happen). But sometimes, the tower speaker starts playing the music even when the headphones are inserted! So, the music is played through both of them! This gets quite embarrassing.

So, my question is - Is there a way to turn off this "tower speaker"?

EDIT : I'm using Windows XP