With my creative sound card I used one of these
Griffin SmartShare headphone splitter?black | Headphone Accessories | Clearance Outlet | TheSource.ca
Plug this in to your green line out on your sound card then plug in your headphones and speaker line into the female ends. Your pink mic plug goes into your sound card as usual. The only drawback is that you need a volume control on the headphones themselves. This setup has always worked for me.