There is a bug in some versions of iMovie that causes intermittent loss of audio in the exported .mov or .mp4 files of some videos.
Here is a fix that is known to work and hopefully will work for your situation. With your project open in iMovie, select one clip and choose ‘Extract Audio’ from the ‘Advanced’ menu. Repeat for each clip. Save your project and try the export again. The new exported file should have all sound back intact.
The procedure works for projects that have transitions between clips as well. One report of loss of audio occurred after the trash was emptied within iMovie, so you might want to hold off on emptying the trash until you are done exporting.