Here is what the walkthrough says about the sound options:
The options menu has:
Sfx option has overall, music, speech, effects and environment volume selections.
Save the changes using the save selection at the bottom of the screen.
If you still are not getting speech after this, then I would make sure that the volume is up on your speakers and that your other sound settings are working properly.
And maybe someone else will have some further advice to offer here.
Also - are you hearing any speech at all but if so is it just sometimes hard to hear? Because I found an old thread on GameBoomers talking about this here.