Ok - I've solved the problem.
I downloaded Handbrake and converted the original movie to whatever default Handbrake had (didn't worry about it). Didn't take long.
Then I created a new project. exported the audio.
OK. Edited the project a little bit.
Exported the audio. OK.
Edited more, worried that something will go wrong.
Exported the audio. OK
Then finished editing and exported the audio. OK
Exported the movie. OK!
I assume the problem is somewhere in the original file - which was created by Microsoft Teams.
I'll just convert anything from Teams before doing anything else now. Which will save some heartache
Thanks to those who offered suggestions