Hi, Matt,
iMovie can sometimes be more sensitive to codecs than VLC or QuickTime.
First, check the link below to confirm that you are using a compatible format.
https://support.apple.com/en-au/HT209029
If all O.K, try these procedures and see if any of them cure it:
Safe Mode
Try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly.
Resetting the System Management Controller
Try resetting the System Management Controller:
https://support.apple.com/en-us/HT201295
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM, although this issue would be fairly rare:
https://support.apple.com/en-us/HT204063
All else failing, try re-rending the video clip to Mp4/AAC (even if the clip is already that format) with the free download, Handbrake. Re-rendering often cures sound issues.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie. It should work fine.
-- Rich
