It's odd and I don't really know the solution. I have never had this problem.
Perhaps these voices only work in 64 bit applications and not in 32 bit. Ironically VC was kept as a 32 bit app with some effort specifically because more voices seemed to work in 32 bit mode. I don't know if that is still true, but now there are a lot of other reasons not to switch to 64 bit because it would break stuff.
The only suggestion off the top of my head is to try running VC as administrator (as in right-click and choose run as administrator), but it's a long shot.
I have tested Ivona demo voices in the past and they worked, but I do recall them acting up under certain circumstances. I use normal Ivona voices without any issues. Some voices don't share well, so you must make sure they are not being used by another program.