I had this same problem with my Turtle Beach Montego DDL.
The audio portion would not play but I still selected next at the volume screen and was able to hear audio for the Dolby test, then the DTS would come up and I heard nothing. It turns out the DTS sample only works with the coaxial output and not the SPDIF. So if you are sure your card support DTS just select yes you heard it.
I was not able to hear audio from mp3's or from non dolby recordings so I went in to kmix and discovered that changing the master volume source to ac3 and selecting the appropriate line outputs for digital output corrected the problem.
Audio playback is working 100% after I do these steps but the mixer seems to loose the settings after being rebooted, not after router reloads. So this would seem to be a KDE or Kmix issue I will have to work on.
Anyway, before you go scrapping your whole install over this try changing some settings in Kmix with an audio CD inserted or some MP3 file playing so you can hear when audio starts working while you are in the mixer settings of kmix, which is easily accessible via the panel on the KDE desktop in the lower right hand corner.
Good luck.