Voice commands are not available if the alert type for calls or alarms is set to vibration.
[Android™ 4.4, KitKat] What is Voice Control, and how do I use it on my Samsung Galaxy S® 4?
Your device can answer or reject incoming voice and ChatON™ calls with the voice commands "Answer" or "Reject"; stop or snooze alarms with the commands "Stop" or "Snooze"; take photos with voice commands like "Capture," "Shoot," "Smile," and "Cheese"; and control the music player with the voice commands "Next," "Previous," "Play," "Pause," "Volume Up," and "Volume Down."
To turn on the Voice control feature, from the Home screen, touch Menu > Settings > My device > Voice control . Touch the slider to turn Voice control on and touch OK. Deselect any of the voice controls that you do not want turned on.
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