If a clip is MONO, it will be heard on both channels IF there are no stereo clips. If so, it will play on the left channel only. To convert the MONO clip to stereo...
Download and install WAVEpad free trial.
In Videopad, right-click the audio track and SAVE AUDIO CLIP AS NEW FILE.
Click the AUDIO tab in the media bin (top-left of screen). Right-click this new file and click OPEN FILE WITH|WAVEPAD AUDIO EDITOR.
The file loads into Wavepad. At the bottom of its screen the word "stereo" should be seen. Click on it and answer "yes" when prompted to switch to mono.
Then FILE|SAVE AS with a new name to a convenient folder.
Back in Videopad, right-click on the audio track and UNLINK FRIOM VIDEO if necessary.
Click ADD FILE in the toolbar and import the file from where you saved it.
Delete the old audio track on the VP timeline and substitute the new mono file.