Audio problem (volume of voices too low)

[Heureka]

As the title says, conversations between characters in cut scenes and in-game are so soft, I can barely make them out. The only way to fix it I found so far, is to put the speaker option to Stereo or Headphones (which isn't a decent solution... I don't own a surround system to then use stereo...).
Did anyone experience the same and knows how to fix it properly?