I also was thinking about voice acting. I don't know if Winter had in mind the Jagged Alliance 2 voice acting style, but I'm suggesting something similar.
I have some ideas regarding this topic. First, because you already have localised versions of the game, it will be hard to have voice acting for all the localised versions. So, to sort this thing out, the voice acting should be in an optional add-on, and in the begining, start only with English voice acting.
I would like some voice acting for the soldiers, like each having a set of vocal replies during combat - the common "yes, yes sir" and other typical lines ("good as done", "ready and waiting", "I'm on it", "One alien less", "Shoot to kill"). Besides, they could also say "I'm out (of ammo)", "I'm under fire", "I'm hit", or "I got you now", "You're disection material" (when an alien is killed), or say something when using reaction fire. A problem is having soldiers from all over the world, so there should be several voices for each kind of line (to fit each actor type) plus several accents to accomodate for each continent/country.
The designers should think about how many soldiers a player will have/use in a game, from start to the finish. If there will be less than four squads, then it would be nice if the soldiers will have a clearer identity, like a file. Fallout Tactis featured a similar system, but the player controlled only one squad.
Some voice acting could be used on the geoscape as well, to hear feedback from your pilots or even base commanders (when you click on a base or craft). Civilians should have some lines as well, like "heeeelp", "we are under attack", "we're all going to die", "my sister is missing", etc.