Why is the iMovie Noise Reduction and Equalizer button greyed out for some videos?

It is looking like there is something about the format or audio codec of the clip that is not compatible with iMovie. If that's the case, converting it with Handbrake should cure the problem.

If Handbrake doesn't cure it, the only other thing that I can think if is to redownload iMovie.

-- Rich

That will happen if the clip has no audio.

-- Rich

That will happen if the video has no audio.

-- Rich

Yes, sometims .mp3 audio can cause issues. As you found, iMovie likes AAC audio.

Glad you got it working.

— Rich

Sometimes when an an iMovie function doesn't work as expected deleting preferences will cure it. To delete 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.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

If still no luck, try putting the clip into a newly created project and see if the Noise Reduction and Equalizer function works there.

Failing that, try converting a duplicate of the clip to H.264, Mp4/AAC with the freedownload Handbrake. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures the issue.

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