I had the same problem suddenly and I now know what the problem was: I had changed the Audio Mode in my Realtek HD Audio-Manager to 7.1 Surround-Sound. With the 7.1-setting you can't hear any voices in-game, at least for me it doesn't work, I have only Stereo-Headphones. Changing back to normal Stereo-Mode brought back the in-game voices. I guess when you choose the 7.1-setting, the voices will be played on different audio-channels. I personally use the 7.1-Setting with my Stereo-Headphones for boosting up the volume in other programs.