None of my sound effects are showing up

Cleanmymac is known to delete stuff and make the system unstable; if this is the case, just uninstalling it is not enough to restore the system.

You may try running the Combo Update for the same version of macOS you are running; but the only comprehensive fix for a machine that has been touched by this program is a clean install.

First, make a complete backup to a separate drive. Use Time Machine AND/or make a bootable clone using CCC or SuperDuper! --- A BACKUP is extremely important always, but even more so as you are about to completely erase your drive. I strongly recommend making the two versions above.

Second, download the installer from the App Store. If it starts automatically, stop it.

Third, follow the existing instructions for creating a bootable installer in a usb thumb drive.

Fourth, restart your mac holding down the Option key, and select the installer drive you created in the previous step.

Select Disk Utility, and completely reformat the drive as APFS.

Fifth, quit Disk Utility and proceed with the installation.

You now have a clean system. You may elect to Migrate content from your backup, BUT ONLY the user accounts - do not bring back any settings, applications or other files.

Reinstall iMovie, Final Cut Pro and test. You will likely find out that your mac is working much better than before.

I had a similar problem though it may not apply to you.

I had many gigabytes of sound effects from Logic Pro etc. and I found they didn't appear when i clicked on "Sound Effects".

At least, they didn't appear immediately! I had to wait several minutes for them to pop up.

Removing most of them solved the problem.

My point is that they were there but took so long to appear that it made me think they had disappeared.

The jingles are part of the iMovie package. Try reinstalling iMovie.

The additional sound effects content for FCP is not available in the trial version.