If a MONO file is left or right channel only, then that's precisely the way VP will play them. Sliders stay centered.
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If the sources are stereo then you can pan one track's slider to the left and the other to the right and they will not mix.
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If both sources are single-channel centered MONO you will need to convert them to two-channel centered MONO. However, if one source is stereo, this should not be necessary. For example: a stereo vid/aud file on one track and a single-channel MONO audio file on the other. In that case the sliders should be effective for both sources..
Should conversion be needed, use Wavepad. The trial is free. Load a file, click the MONO button at the bottom of the screen, click the chevron at the right of the box that appears, click STEREO, click the X to exit.
Now locate the saved file on your PC, import it into Videopad,. Before replacing the original video file's audio, UNlink audio and video via a right-click on the audio track -
-and replace the audio with the file saved by Wavepad. Then adjust the sliders far left and right.
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If you attempt to load a video file into Wavepad it will prompt to continue as audio only. Say yes. Then, per the above procedure, save the audio only file in the format of your choice.