Yeah, go to Map -> Player Properties,
Then change their race from there. What it's set to, is the UI they get in game.
Edit: Oh and as for voice etc, you can record your own sounds or import and set it to the default sound you get when you click that unit. Not sure exactly how that's done tbh ..
Music just import and use a trigger to play it,
And I have no clue about UI, because I've never even worked with it in WC3 =3
Hope I helped?