Sounds like you have a configuration issue. Check to see if your speaker set up in windows is set to Stereo or some type of surround sound. Set it to stereo and see if you now hear voice overs. I haven't messed with 5.1 in this game, but I do know from other games that sometimes voice overs are only output through the center speaker. If you are wearing a headset but have 5.1 set, you wont hear those voice overs (as well as other sounds.)