Imported audio distorted with a reverb-like effect

iMovie is a bit more sensitive than QuickTimePlayer to slight aberrations in audio format or rendering.

Try converting your troublesome clips to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to be Mp4/AAC. Handbrake will convert to Mp4/AAC by default. Re-rendering often cures audio 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.

-- Rich

Try deleting/resetting preferences. To delete/reset preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Possibly you may need to reboot to set the changes.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

-- Rich

You are very welcome. The ones yesterday may have been used in a different project, possibly? Sometimes glitches are confined to just a specific project.

—Rich