Could it be those digital extras that sometimes come with album purchases?
Download and use
OmniDiskSweeper. It will provide a sorted list of what's consuming your space.
If you run it with sudo (As shown below), it will include some system files that it woud not normally have access to scan. That is a more accurate representation of what's consuming your drive.
Code:
sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper
Another option is to use this terminal command
sudo du -d 1 -x -c -g /
I prefer to redirect it to a text file (this puts it in your Documents folder
sudo du -d 1 -x -c -g / > ~/Documents/du.txt
Like the sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper command, it will scan all directories, but produce a text file as opposed to showing the results in a window