Hi,
As you have found, this appears to be an audio issue. When you try to export, are you getting an "lpcm" error message? Other users have reported that. In such cases the only successful export that can be done is with the setting Best Quality (pro res). LPCM is an audio codec, and it seems that somehow, in some cases, iMovie erroneously identifies clips with that codec. It may be something with the clip, or your camera settings, or some issue with iMovie, I don't know. The vast majority of users have not experienced the issue. Best Quality (pro res) exports in the lpcm audio codec, whereas all of the other settings export in the AAC audio codec. That might be why Best Quality (pro res) is successful when the other settings are not. It is a pesky issue and so far I have not seen any solutions reported. Sometimes it spontaneously resolves and then returns. If you detach and delete the audio, you probably will be able to export your video at other settings, like you can with the soundless still image -- which confirms the audio issue.
You have tried the remedies that others have tried, also with no success. Working with a duplicate, you might try converting/reconverting your video clip to Mp4/AAC and see if that will then export from iMovie. You can convert it using the free download, Handbrake. You can also try re-encoding your duplicate clip by control-clicking on it on your desktop and, in the drop down menu that appears, selecting "encode selected video files" and see if the re-encoded clip will export in settings other than pro res.
You can also try booting in Safe Mode (see Help Menu) and then, after everything loads, immediately shutting down and rebooting in normal mode. That might clear out some conflicts that could be causing this. Run the first aid program in your Mac's Disk Utility app, found in the Utilities folder and see if that helps. Try exporting different clips and different audio in different projects and see if you can see any correlations with what exports and what doesn't. Also, was there anything that happened 4 months ago, when the problem started happening, that might explain it? Installation of a new app maybe?
Good luck with this. Perhaps someone on the forum who has managed to solve this will chime in.
-- Rich