I've got a 3790T with European maps. With the original installed map source the voice command is working o.k.
As soon as I switch over to the other map set the voice command is still working but as soon as I try to "find address" an error message comes up which tells me that this is not availbale.
I have the same problem with my Nuvi 855. The voice command works for all functions except destination direction. A message comes up saying that, that feature is not available and that I have to type the destination selection. I was disappointed but assumed that this was a deliberate aspect of the software design. Now that I have seen your post, I am beginning to think that it may be a "bug" in the program.
Depends on the mapset. Some custom mapsets lose this feature. It's to do with files used and converting files. Reverse converter often has this problem. It can also be caused by some unlocking techniques... can't tell you much more without more details.