There are a few things to check if you're having issues with being unable to hear voices. Firstly, if relevant; Voice will only come out of the central speaker if you have a 5.1 setup. Make sure your speaker setup is correct in Windows settings.

If this isn't the case; there are a number of things to check.

1. Check your sound drivers are up to date
2. Check the sound and voice setting in the ingame options are enabled. From the ingame menu, click Settings, then Sound.