Low voice volume, high sfx and music volume on 5.1 through S/PDIF

[Tresty]

I have a home theater 5.1 system connected to my pc through S/PDIF. Everything is working fine, except the volume of the voices in movies vs the volume of the sfx/music.

The problem is also the fact i have no control over the volume of the sound from my pc, the windows volume, the bs player volume, nothing controls the volume, just the remote control from the home theater.

Is there any way to get the central speaker (the one i have read is responsible with most of the voices in movies) louder without getting all the other speakers louder if I use S/PDIF?