Today, I stumbled upon a mystery: I ran across a song that iTunes refuses to play, so I decided to investigate a little bit. To find the file containing the song, I ctrl+click on the file (or right-click) and selected “Show in Finder”. It turned out that the file referenced by the song has a .nib extention instead of .mp3. Fortunately, next to that .nib file is the .mp3 file, so I double-click the .mp3 to import it back into iTunes. Finally, I selected the unplayable song in iTunes and hit the “delete” key and chose “Move to Trash” to get rid of both the song and the .nib file.
After this incident, I poke around a found a dozen of .nib files within my library which I used the same method to manually correct the situation. I appreciate any explanation for insight into this problem. In case you have not noticed, I am running iTunes on Mac OS X 10.4.9.