First, make sure that you are running current version iMovie 10.1.11 with Mojave.
Sometimes export problems can be cured by deleting preferences. To do that restart your computer and open iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen you old library to get back to your projects. Now see if you can export audio without the popping sound.
You can also try putting your media into a newly created project, to see if the issue is confined just to your original project.
Another procedure that sometimes helps is to boot up in Safe Mode (see Finder help menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. See if that cures it.
All else failing, you can try redownloading iMovie.
-- Rich