hi phoenix_one, the headset should be used to replace the speakers ,as a set of headphones does in a hi fi system .... so if you have 4 speakers on the soundcard (2 front 2 rear)then when you want to use the headset, plug in to where the front speakers are . you could buy a splitter jack (splits one socket into 2) ,that way you could have speakers and headset all plugged in at the same time .
The only thing is that 'voice' forms part of 'sound' in the audio output. Your card will be processing the signal and placing the audio to the appropriate speaker, left and right, front and back. 'Voice' will have a tendancy to be directed to the front in most circumstances but it will also be combined with 'sound'. The only way I can think of you doing this is to have a card and set up that accomodates a central front speaker. This you probably could replace with your headphones, but in all of the suggested options you would lose the 'surround' effect especially if your 'phones are of the closed back type.