I’m quadriparetic, so voice control is very important to me. I am currently using a method Will Poirier developed that involves Siri and ifttt to control smartthings functions. Other people are using hardware options, Like Ivee and Ubi, and android users have a whole different set of available options. There is also an official smartthings voice control app for the Samsung gear S smart watch.
We have a pretty active topic that discusses all of these, so you might check that out.