Its alright, That be be a hell of a mod though, both on standard half-life 2 and Gmod.

All good, sorry I cannot really help further

I think I've done all I can in the settings, I suppose it couldn't hurt to look again though.

Without singling out the npc voices sounds themselves, I would not know how else to do that. Addon idea I guess. Have you tried adjusting the audio settings in game?

As in to make it so the voice lines are more audible, half the time I can't hear them over the subtlest of background noise in the game.

Louder in what way? I am trying to better understand the question.

Dammit, do you know any that do?

It shouldn't

Does this make them louder?