If you have an Alexa-enabled Smart Speaker, you can use it to control the Xbox One's voice when it's part of your Smart Home setup. On Xbox One, go to Settings> Kinect & Devices> Digital Assistants and check the box next to Enable Digital Assistants. Then install the Xbox capability for Alexa. If prompted by the ability (a skill is like a voice application for Alexa), enter your Microsoft account credentials. Pair the console with your Alexa account and start speaking.
The standard commands begin with "Alexa, tell Xbox to …" or "Alexa, ask Xbox to …" and then you can add Pause, Resume and Volume Increase, Turn Down, Turn Off, Start [game or app name] or "Alexa, ask Xbox what I can say" for more feedback. If the Xbox is part of your Smart Home setup, you can skip the Tell or Ask part and speak more naturally to at least pause, resume, turn off, or adjust the volume. The complete list of commands can be found here.
If the Xbox One is the only media player you use for your Alexa Mobile app, the Smart Spokesperson knows that your subsequent commands are usually for the Xbox only. So you can try to say just "Alexa, Pause" or "Alexa, play" (or continue or jump forward, etc.)