Improved to some extent, but ultimately not an instant classic as its original predecessor was.
The graphics aren't any better than the previous games and are even going to the childish side. Obviously if you're looking for a beautiful game, look elsewhere.
As for the sound quality and voice acting, from what I've heard in game the ambience sound is pretty good and the voice cast also meshes pretty well.
The standard game play is just like any other real time strategy games on the market, although this game can boast the new Global Domination mode. This mode, although innovative in some ways, lacks real depth and gets awfully tedious having to battle out every turn. It will allow you to enjoy it for some time and then it gets old. The skirmish mode is not all that exciting, but it gets the job done. Empire Earth III also has a more complex diplomatic system than the former games, but it's still not as fancy as let's say Europa Universalis.
So to sum this up, I wouldn't recommend going out and buying this unless you've run out of games in this genre. If your looking into the Empire Earth series, I suggest you look to the first in the series as I don't know if it was just me, but I thought the 2nd didn't do honor to the Empire Earth name and now the same for number 3. Maybe they'll have better luck with IV, who knows?