How do you do that on the PC?
You can't I think. (I have an X-Fi Titanium and I'm not using SPDIF)
There might be a way to output Dolby Digital at the game level. Maybe in the Audio options or in an ini file somewhere? It wouldn't help to process the mix outside of the game, unfortunately; post-conversion transcoding wouldn't fix the mix.
You can output Dolby Digital through HDMI on the consoles for the games that support the codec, so I would assume there's some way to achieve that on PC.
EDIT: I'll go into detail on my current theory: The game's DTS mix is bad. Maybe they'll patch it. Unfortunately DTS is usually the default audio codec used to create LPCM or stereo mixes.
DTS -> Speakers = bad mix
DTS -> LPCM -> speakers = bad mix (default on PC and PS4, I'm assuming)
DTS -> LPCM -> Dolby Digital -> speakers = bad mix (using a sound card to post-convert)
Really the only way to get the good mix is to start with Dolby at the game level.
Dolby Digital -> anything = good mix