revial: The patch is supposed to add separate sliders, and is supposed to be out next week.
While I never had an issue hearing the voices, it's obviously going to be different, depending on one's hearing.
Many audio cards have audio leveling in one way or another (increase sounds of lows, decrease sounds of highs). If yours does, you could try that as an interim fix for the time being.
If your audio card has a utility that allows you to vary channels individually, you could try that as well.
I'm using a 5.1 audio card (built-in AMD HD Audio) and 5.1 headphones (Turtle Beach).
I can't really tell if the surround sound is even working. In the beginning of the game when I was climbing up the ladders following Foltest, he was talking and talking but I couldn't hear a damn thing.
Also sometimes I hear something like it's close to me, yet it's actually far away.