How to configure VLC to turn mono signal into stereo?

I've got some video files with just one (left only) audio channel in them. I'd like VLC to duplicate the channel to make it sound in both ears. Is this possible?

You're thinking of it the wrong way round…. It is stereo, even though the actual sound is only in one channel.

What you need is for it to output in mono which you can do from the Audio menu, just pick the Left channel & it will output that to both sides.

Top Menu - Media - Convert - Add (Choose your mono file). Click Convert/Save button. Under Settings, select Convert and in the Profile drop down, select the audio type you want, (Audio CD) for example. Now click the Wrench Icon. In the Encapsulation Tab, select the extension/container type you want, (WAV) for example. Next, click the Audio Tab. Make sure Audio is ticked and in the Encoding Parameters Tab, select (2) in the Channels Bar. Now select the Filters Tab and scroll down and tick the Stereo to Mono downmixer option, then click Save. Now Click Browse next to Destination File to select where to save and give it a name you want, then click Save. Click Start to turn your Mono file into a Stereo file :-)