Custom Sound Set

Can I create a new set of unit sounds, a new "what1", "Yes2" and so on, WITHOUT changeing the existing sound?

I believe that you will have to edit your model in .mdx

Or you should look at the campaign Rise of Krill. One of the heros has a new sound set

You make your sound, and then import them into the war3patch (Because then the old sounds will still lurk around) with the same names as the original sounds.