Out of the blue Sound Effects have gone missing in Final Cut Pro.
The sounds are still on the computer where they belong: /Library/Audio/Apple Loops/Apple/Final Cut Pro Sound Effects.
Several users have complained in past years and no real solution have been offered. Even some people went to the length of trashing all prefs, trashing the sound package, re-downloading the sounds, re-installing FCP... To no avail, the stuff of nightmares.
I found that closing all FCP Libraries and creating a new FCP Library... Boom! The Sound Effects appeared all of the sudden. From there, re-opening my working project, closing the empty newly created Library, and yes the Sound Effects are still there.
There is a glitch somewhere that makes Sound Effects disappear and re-appear. At least this workaround works for me.
I also noticed that the Photos folder was also missing altogether from the list. It showed only Logic Pro X, iTunes and Sound Effects (empty.) After the "re-trigger" move described above, the Photos folder re-appeared.