I was thinking and I made a bit of a realization.

 

With the exception of Roxas, nobodies usually look and sound like their somebodies, right? Xemnas is basically the nobody of Terranort, right? And when Terra was norted, I can safely assume that the only things that changed (physically) where his hair color, eye color, and skin color.

 

So here's the thing, if Xemnas where to turn back into Terra, he would still have that slow and deep a** voice... I can't picture Terra coming back now.

Not likely. Ansem and Xemnas where created differently than most heartless and nobodie like Roxas was. Sora didn't get Roxas's features when he merged with Sora, so I doubt the Terra will be any different.

If Terra does remerge into an older version of himself, I doubt he'll speak slowly. Deep, sure. Besides, his VA in BBS wasn't exceptional, anyway...

This was a slightly different transition than the others.  Xehanort had used Terra as a vessel, whereas the others had been subjected to losing their hearts and had transformed into Nobodies.  There's a bit of a difference there and, had Terra only become a Nobody, I think his voice--along with his personality--would've stayed intact.