Each content library uses a "Folder" that contains files. If you create a copy of a content library, by default it will use the same folder as the original content library.
If you go to the Settings tab of the content library, you can see the name of the Folder.

Files of content parts are located in

Files for questions can be located in these folders:
https://example.anewspring.com/images/Folder_name/ (for images and Plus Editor files)
https://example.anewspring.com/sounds/Folder_name/ (for audio files)

Files can only be loaded for learners if they are located in the folder that belongs to the content library. If a link in a content part of question refers to another folder, it can happen that a learner can't see the file.
This can also cause problems for the author if they want to make changes in an existing content part. They will get an error when saving the content part if it contains a link to a file in another folder.
This problem mostly occurs when a content library was exported and then imported into another content library, without filling in the folder name of the original content library in the Old folder field. If you fill in this field, all links will be changed to the location of the files in the new content library when doing the import.