Just a quick article to help others who run into this frustrating issue.
I was working on a video project and while the Windows generated clips played perfectly in VLC and Quicktime Player in Windows and MacOS, once imported into iMovie the first 1, 2, or 4 seconds of audio was muted.
How I fixed it
I downloaded Handbrake and converted my files to use the AAC audio codec (I selected maximum quality for the audio and it sounds fine). The output files were about the same size and I was able to Open the contents of the iMovie Library file and replace the clips under “Original Media”, though it glitched once I added new clips, I didn’t have to recut the movie!