I know that voice search on the Now TV Stick currently only works with the Now TV app and it would be quite some time until results include content from other apps, if at all. But as Sky Store is a Sky app, I suggest that it would be useful to include content from Sky Store in search results, as long as they were labelled as such.
If this has solved your issue please mark this as an accepted solution.
Those little green stars by my name mean that I'm a Community Contributor - not Now TV staff. I just try to help out if I can.