[Zubersoft]

It is certainly possible to use the same audio file across many different songs/PDFs. If Settings->Storage->Copy Audio Files is enabled, and you have "Create Subdirectory per Song" enabled, then this feature won't really work - a separate copy of the audio file will be stored in each song's subdirectory. If you got rid of the subdirectories, or if you manage your own audio files outside the storage location, then it should work. If you manage your own audio files (leave "Copy Audio File" unchecked), then MobileSheets should use the files from their original location, but it means you need to import them from the device storage and not from the cloud. This gives you more control over how they are handled, but you need to make sure to select the option to restore to the original file locations if you ever restore a library backup file.

Lastly, you asked if unused files are placed in backup files - the answer is no. A file is only included if the active database/library references that file.

Mike