Split Clip is dimmed when I try to split an audio clip in iMovie.

The only time that I can induce a dimmed out Split Clip option is when the playhead is not positioned somewhere in the selected audio clip.

If the above is not your situation, then in addition to trying a new project, try redownloading iMovie. Move your iMovie app from the Applications folder to the trash, but do not empty the trash. Then sign in to the app store and redownload iMovie from your account purchases list. If unsuccessful, you can move your iMovie app from the trash back into the appplications folder.

-- Rich

In a pinch, try duplicating the clip (Edit/Copy, Edit/Paste) so you have two identical clips, and then trim back each clip to get to the split point that you want.

Also, try doing a Split Clip in a new project and see if it works there.

— Rich

Usually the deleting preferences procedure cures glitches like this. Even though you already have deleted preferences, try it again, as sometimes the second time will work.

Sometimes updating iMovie and the operating system to most current cures issues.

-- Rich