If you use the HDMI output for sound you will only have stereo sound with games since only PCM encode is supported. For the optical output it depends on the soundcard settings. Some soundcards require to use the "speaker" output even if you want to use a digital output (ASUS Xonar) and set it up to use the dolby encoder. Realtek HD Audio chips don't need you to use the "speaker" output but you have to set the audio output format to "dolby digital live".
If the dolby encoder is bad (or even broken) there can be issues with the loudness of 3d sounds in games.