I'll try to explain Mutator/Mods.
First, you never want to edit the existing packages. Say the game ships with KFSuckyVoiceActing.uax (where .uax is an unreal audio package). If you make changes to that file, you will no longer be able to play online (offline will still work, though). Why? Because any server you attempt to join will say "Hold on, your version of the game isn't the same as mine. Go away."
This is where Mutator/Mods come into play. A mutator is a minature mod, of sorts, that can only change small aspects of the game. A mod is, well, a modified version of the game. Either way, these are new files/packages/code that you've created and integrated one way or another into the base game. In order for others to enjoy your Mutator/Mod, you have to have a server running them and everyone connecting must either have the files already or download them from the server.
An example: You create a new audio package called KFBetterVoiceActing.uax. You must then modify the game code so that any time the old KFSuckyVoiceActing.uax package if referred to, it instead uses your new package. Once it's modified, you have to put all of the modified code/files on a server and find some way to get the code/files to everyone who will be playing on that server.
As for making an audio package: I made one earlier that contains all of the Resident Evil 4 Merchant sounds, so I can give a brief tutorial:
1)Have your sound files. As zYn listed below, they must be in a specific format in order for KFed/UnrealEd to accept them. I'm fairly certain that, on top of his listed requirements, the sound file itself must not have spaces, punctuation, or anything other than letters and numbers.
2)Open the editor. Go to View->Show Sound Browser. Then go to File->Import. Find your sound file, and try to import it. It should bring up a window with 4 text fields:
File (your file's location)
Package is the package you want to place this file in. If you provide a package name that doesn't exist, one is created. This is where, for example, you'd type in KFBetterVoiceActing. The first file imported will also create the KFBetterVoiceActing package, and every file you import later will be placed inside the package as well (provided, of course, you tell the editor to place each imported file into the package).
Group allows for organization within a package. As an example, you could have a "Snarky" group for witty comments and a "Desparate" group for when someone is begging for help. They're unnecessary, though (and add a teensy bit of code work, eh).
Name is the name you want to give the file within the package. It's usually better to make names descriptive and organized.
So, simplified: Make sure the file is in a reachable location (C:\Voices\), make sure the file is in a proper format (preferably .wav, "mono 16bit 44khz or lower", and has a short letter/number only name), create a new Package with the first file imported, and then import the rest.
Sorry, I'm rather new to this myself and can't explain it perfectly. I can tell you this, though: Changing the voices is going to be REALLY, REALLY ANNOYING. Making a sound pack is a breeze, but sheesh the in-code calls for voices / sound effects are scattered all over the place. Heck, the "Trader" doesn't even exist as an entity. It's just a bunch of GameType calls for sound effects coupled to a few, uh, sounds spawned from a player when they are shopping and I think a few other calls/places. It's not worth modifying unless you're going to be doing some other major changes, I think.
No voice acting required! i'm from northamptonshire.