OK so first thing is first. Yes the final release will be modular. Every civ I have worked on since BtS was released has been modular and I see no sense in stopping now.
As for the Australian Diplomacy screen. Honestly I had never noticed that there was no music. I will try to fix it for the next release. However none of my civs have Custom music, so it will be "borrowed" from another civ.
The voicing done for the units, aside from the Romanian sound set all are the original Fraxis released sounds.
Hope that answers all your questions for you.