Thanks, I've just exported a clip to .wmv and the sound comes through fine, I'll do an MP4 now.
Here it is, I've included an exported mp4 with no audio. https://drive.google.com/drive/folders/1UdbOsJcDU2_jmtUpa7EevvaO6yL-d6xX?usp=sharing
The original video is silent (intentionally) until about 1 min.
Also I realise that I may have never exported to MP4 before so I may have had the problem all along. I've just started recording mp4 from a Blackmagic Atem Streaming Switcher.