Only two ways to get audio the way you want it.
1) Get a splitter to split the audio from the back of the PC or 2) when you want to use the headset plug them into the front audio and it will disable the back ones. Windows doesn't really suppot having audio coming out of two different ports.
Was weird though because on my old Core 2 Quad ASUS Motherboard I had my turtle beach plugged into the Optical, and then my speakers in my normal speakers out, and even when i had the speakers set as default i could still get audio though my turtle beach though the optical but with my AMD rig I can't do that anymore.
Not sure why that worked that way before.