Hi there, this is usually to do with your sound setup, can i ask what speakers you have? Try setting the sound in-game to stereo and see if that sorts it, also go into your playback devices and right click on 'speakers > configure speakers' then change it to stereo there.
I'd like to know if you have 5.1 speakers though as this was happening to me with my 5.1 creative setup. I have the HD audio manager where when i plug a speaker cable in it asks me what i had plugged in and not realising it had detected my center speaker as something else so i was never getting real 5.1 hence no voices in games as they tend to come out of the center speaker.
Anyway try what i said if you have standard desktop speakers and if not let me know