If you are using a 5.1 and up setup this is an issue currently. The workaround for it would be to manually set your speakers to quadraphonic or stereo in your sound panel.
Logitech Headset users: If you are using a G35 or G930 simply using the toggle switch on your headset to switch between stereo and back to 7.1 will fix the dialogue issue ( note you'll have to be in a dialogue\cutscene for it to work.)
It is a sound driver issue. I get it every time ruddy windows updated something. Just need to set you sound output to use stereo sound and it will work again
Logitech Headset users: If you are using a G35 or G930 simply using the toggle switch on your headset to switch between stereo and back to 7.1 will fix the dialogue issue ( note you'll have to be in a dialogue\cutscene for it to work.)