The main problem is, in my opinion, that you aren't a professional voice actor. The people who did the voices for KaM, are probably (I'm not sure) fairly professional, but you do this in your room. That's a big difference. You should first try and think of a voice you think fits the unit you want to do, then practise it a little, and finally, like Lewin said, do multiple tries for each sound file. And I'm not sure what you're using, but if you aren't using Audacity, you should. You should keep quiet for a few seconds between each try, so it leaves a lot of room for you to cut it. Right now, there are some files in which at the beginning, the words are already being said, if you get what I mean. This may not be a disaster, but it would sound a little awkward when playing the game. Doing the voice acting is harder than you'd think.
Anyway, I think it's really good that you try doing this. Right now, I fear it wouldn't be good enough to be implemented in the game, but if you keep trying and improving with the tips people give you, like you're doing right now, you'll improve big time. Thanks for trying to improve the game!
~ Bart.