Once you have uploaded or recordeda voice-over, and added it to your video, you can trim it to keep only an extract of it.
In the Audio tab in the left menu, in the Voice-over window, open the voice-over editor.
In the timeline, you can drag the ends of the recording to cut it.
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Dragging the left end to the right will cut off the beginning of the recording.
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Dragging the right end to the left will cut off the end of the recording.
It is also possible to split your recording into two separate pieces. Simply position the cursor at the exact point where you want to make the cut and click on the scissors icon :