Official comment
This is an issue due to Apple's 11.5 update which changed the file permissions; there are two work-arounds while we wait for Apple to correct the issue:
1) Use this command in the Terminal to change the attributes:sudo xattr -cr <filepath to audio files>
So if your files are all located in your user's Music folder, it would be:sudo xattr -cr ~/Music
You would have to do this anytime you add new music to the folder until Apple resolves the issue.
2) Instead of exporting to a FAT32 formatted USB drive, use an HFS+ (macOS Journaled Extended) formatted USB drive instead.
Thanks for your understanding.