I found the solution. Export the video even though only the audio works because it still syncs with the slides. Import this into Videopad as video. Then import the video as an overlay as video 2. So you end up with 1 video and 2 audio recordings. Move the audio recordings to sync with each other and save. It works! It was always going to be impossible to export as a video a pptx presentation which already contained a video: should have realised that at the outset!