part of the sound track disappear when it is added into fcp

Whenever something goes wrong in FCP, deleting its preferences is worth a try.

To delete preferences, hold down command and option while starting the application and click Delete Custom Settings. The application will start with default settings and open an "Untitled" library. Adjust the settings, reopen the library you were working, and see how it works now.

If the problem persists: Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

Not enough information for us to do much...

What versions of macOS and FCP?

What type of file?

Does this happen with other audio files?

Does the problem occur in the browser or when adding to the timeline?

Please post some screenshots.