There are some NPC's you will never hear talking - whatever was needed just didn't make the final product and wasn't fixed in the patch.
Has been quite a while since I played but I'm sure there were a number of non-town NPCs that I never heard either while I think other players did - Hopefully someone will know the solution. It didn't really bother me, in fact there were times that I wish I could have turned off the text too! (especially Sebastio)
Wyv