The voice command actions are very limited in the Android Operating system apart from Moto x their is no other device which support Voice Commands outside Google search or Home screen, Now Google to Integrate voice commands on Android operating system. This change is done by the Google with the help of Ok Google hot word and voice action supports across the Android operating system even inside the Application of the Android smartphone.
According to the reports, Google is planning to Integrate the voice commands on Android operating system by using word called Ok Google. Whether we will be in Search Application or Home screen or inside any application like calender if you say just Ok Google add an event then this application automatically opens the new entry and enters the new event what you have said. There is no announcement that this voice action might be applicable for the all applications or else only for Google applications.
This will possible for the Google to start with its own Applications and to provide some API and some other tools to their developers to Integrate voice command functionality. Google is also planning to Integrate a new button which will be replace by home button to Google button. If you click on that Google button it will goes to Google search as well. This will be possible only with the Nexus devices and also Google play devices, Because for other manufactures they have their own skins and home button.
But their is no word officially announced by the Google that these changes will be done or not, It is just a rumor circulating all over, Their might be a chances to do all such things by Google. Stay tune for more information.