Missing audio in FCPX

First of all, delete preferences: hold down command and option keys while starting FCP X, and click Delete Preferences.

Then reopen your library and try again.

Do this again whenever there is a crash, or the application is not behaving correctly.

Any improvement?

Adding a clip to a project and then detaching and deletingh audio in that project should not affect the clip itself.

Nor should it affect the clip if it is added to a compound clip.

Changes to compound clips affect other instances of the same compound clip, but not the clips that are used in the compound.

If some of that happened, then it is a bug.

Can you use the Open Clip functionality NOW and see if the audio is really missing in those sections?

(Note: when you use Open Clip, you should see video and audio separated like when you do detach audio; that is normal)

Perhaps you accidentally used the Open Clip function and edited the clip itself?

That would explain it.

Final Cut Pro is a nondestructive NLE - as are all of the major NLE.

This means that the original files are never changed.

Each clip in your library refers to a media file, but it is not the same as the media file.

If you select a clip in the browser and do Clip->Open Clip, you open a timeline for that clip; you can, for example, delete the audio, change the volume, even add effects. This will affect the clip when you use it somewhere (typically in a project), but will NOT change the media file in your Original Media; nor will it change the optimized version of it.