If the source video has a soundtrack, it will play alongside the voice-over. If the sound interferes with your commentary, you can make the existing soundtrack sound quieter so you can use it as background music or even turn it off.
Select the source soundtrack. It should be right below the video track. Then click the Clip Properties button. Drag the Volume slider left to lower the soundtrack volume. We recommend a volume of around 20% of the original.
To disable the source audio completely, click the Mute track button to the left of the track. If you decide to turn the background sound back on, click the same icon again to unmute the video.