Well, first of all, you need to have a ready soundset. If you've got your own soundset, it's easy - download Ghareth NPC from http://www.pocketplane.net/oneday
, take a look at his .tp2 file and his audio folder, and arrange your files appropriately.
If you haven't got your own soundset, and you want to use existing game sounds, you can edit your .cre files via Near Infinity - if you know how to use it. Your cre files have fields for every sound(HAPPY/UNHAPPY/BORED) - you only have to edit these lines. The downside is that you have to know which sound is which; and there're a lot of sounds in the game.
You can just create your .cre files anew: create a character in the game(in the actual game), assign him a soundset, then quit the game, convert .chr file to .cre, and voila - he's no longer mute.
That's all I can say on the matter, sorry.