Alright! ^_^ First thing first,
This free audio editor, Audacity. You'll need it to cut that silence at the start of that sound clip. It's what I used to get rid of the silence and you can do it fast and easy. You can do all sorts of extra effects as well, such as reverb, sound/noise reduction, ectera. (It helps the quality of your game out) After you get this audio editor (or anything that lets you edit sound), we can move on to the RMXP stuff.
-Replace the old sound file with the new one that doesn't have that 1-2 second silence at the beginning in your SE folder.
-Make sure the attack and animation are both connected to the person you want it for. (Example: Your most recent upload didn't have the "This is the end" voice in animation 109. So whenever you attacked, there was no voice there.) Make sure the Fatal Strike animation is on the correct slot where the voice is located, and then place the sound into "Target Animation" and not the "User Animation".
-Make sure the voice is above all other commands in the animation event page. (Not sure if it helps, but it certainly doesn't hurt!)
-Try it. =p