So to clarify, your issue isn't that you don't get audio from DTS/DD, but that it only plays in 2 channel instead of 5.1?
If so, your problem is the way you have the audio connected to your receiver. A SPDIF optical cable needs to go from your PC directly to your receiver. When it goes through your TV, your TV is taking that DD/DTS signal and downgrading it to 2 channel. TVs (at least most) cannot pass untouched DTS/DD. You'll then need to set that SPDIF connection as your default sound device under your Windows sound properties. You will still only hear 2 channel audio, except when you play something that has DD/DTS. Windows will (you may have to tell the program to do so) pass the DTS/DD signal untouched to the receiver and the receiver will decode and play in 5.1. You'll know it is working because your receiver will light up in some fasion, typically with the DD or DTS logo.
If you want everything in 5.1, then you need a sound card that supports DTS Connect or DD Live. Either of these will encode, on-the-fly, a 6-channel analog audio PC signal to 5.1 DTS or DD that a receiver can decode.