Can't see how the two different outputs are emitting separate audio sources... Is it like receiving audio from an application through the speakers and audio from an MP3 through the headset? If you got one, there must be a setting that addresses this in you audio card settings. Most external speakers also come equipped with headphone jacks and if all else fails you could pick up a splinter like this one
to plug into the green jack on the back of the computer, so your speakers and headphones share the same source. Hope this helps!