The Voice Control button is displayed by choosing which one of two Onscreen Shortcuts is displayed.

The one you choose appears over most menus.

Select the button to display at Settings > My Profile: 

 

If you're prompted to save the setting, don't forget to do so.

When set up, the button is superimposed on the lower right corner of most menus. 

Refer to the FAQ "What can I do with Voice Control?" for more information.