You see subtitles when speaking with him? If you don't get any response out of him then it's the dialogue bug introduced by 1.6.
However if you get him to say some things, or you see subtitles and there's just no audio, then you most likely have your audio formatted improperly.
It needs to be mono channel, 16-bit, 44100hz wave, or xwm. (Or packed into a fuz)
So long as you have the voice type selected in the traits tab of the actor window. Also make sure that you have the right file structure set up, and have named the folders appropriately.