If it's talking during a video playback, then the audio channel selected is probably the one that provides descriptive information. Only some channels offer something like that.
If it's describing how you are moving the selection on the Roku home screen, the Audio Guide has been enabled. Hitting the * button four times rapidly will toggle it on/off. It can be completely disabled in the Settings menu under Accessibility/Audio guide.
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The options button is located just below the directional pad and at the right hand side of the Roku remote. You can also turn Audio Guide on or off within the Accessibility category of the Settings menu.