I strugled with this problem a bit too. A lot of posts said that I should make a lot of stupid changes to my computer.
Some said to try and push my jack a little bit further inside the slot. I descovered that all you have to do is pull it a bit. Juggle the jack of the headphones a little bit in and out of the slot while playing a movie. You will hear the voices when it is plugged in not too much but not too little either and it will surely work.