When I first played with VideoPad, I wasn't able to mute the original audio track. If you look in the lower-left corner, there's a section called 'Audio Track' and to the right of that is a squiggly line representing the audio track that came with my original video clip. If I right-click the squiggly line, I can choose to 'Unlink Sound Clip(s)' and it moves that audio down to the row for 'Soundtrack 1'. Then I can right-click the squiggly line in 'Soundtrack 1' and delete it by choose 'Remove selected clip(s)'