Some fairly common monsters in the module, such as troglodytes and stone lizards, have no voiceset at all. I realize it may be difficult to choose appropriate ones for some of the EfU custom monsters since all you have is a limited selection of Bioware-made voicesets, but I think some more effort could be made in this area regardless. There are a couple of good reasons for this in addition to added flavor, and both of them have to do with stealth.
Firstly, when you are stealthed near NPCs that have no voicesets, you have to keep a constant eye on them using tab to see if they have detected you. This is because if they do detect you, they don't issue the battlecry appropriate to their voiceset (since they have no voiceset). They simply begin to run towards you. Often, it's difficult to tell even then that they've noticed you if the monsters normally walk around in the area. In the worst possible scenario, you don't notice them having noticed you before they've taken their first swing at you, although your character certainly would've heard them running towards you, issuing cries to others, and so on. A bad camera angle can very well cause this if for instance a wall of some kind blocks your view (even if it didn't block your character's line of sight, or hearing). Only a handful of monster NPCs would be intelligent to attempt to conceal the fact that they had indeed noticed you, and an even smaller bunch would be skillful enough to do it properly.
Secondly, when the monsters are stealthed and have no voiceset, you don't necessarily notice them coming out of stealth to attack you since they don't "say" anything (or grunt, growl, whatever) when taking swings at you. What happened to a character of mine once was that a stone lizard came out of stealth right in front of him and scored a hit that took a fourth or a third of his HP; and I didn't notice anything except the HP bar falling. For a full round or so, I looked stupidly at the fallen HP, thinking "Wtf", and "Why can't my character move forward". I didn't notice the lizard because my camera angle was such that my own character blocked most of the view, and I wasn't tabbing. It was only after the lizard scored a second hit that I realized what was happening.
So to conclude, the NPC monster voicesets aren't just some cool things that add flavor: they also serve as auditory warning signals whose absence can make the NPCs deadlier than intended. That's why it should be seriously reconsidered if leaving them away in some instances is really wise. If you think that this is how it should be in the first place, alright, but I don't agree with that because of the limitations that the NWN interface imposes on what you, as a player, can see and notice.