To be able to record your voice over a video, you will have to connect a microphone to your computer. If you are recording on your laptop, chances are it already has an integrated microphone. When you start recording with the built-in Windows record, simply press the Unmute microphone button on the small widget. This will record anything you say to your microphone until you stop recording. Of course, if you need, you can mute and unmute the microphone input at any moment while recording. This will not affect the recording itself in any way visually.