You loading a file into Videopad - or using WAVEpad?
If the former, try a right-click on the audio file and Edit in Wavepad.
At the bottom of the WP window, choose MONO.
X out of the app and it should replace your Videopad file.
If there's no Edit In Wavepad choice via a right-click just open your saved file in that app, make the MONO change, then import into Videopad.