Verify that entries in iTunes are actually on disk
Choose Edit > Select all.
Now choose File > New playlist from selection. Call the playlist "Exist". Only the listings that exist on disk will be added to this playlist.
Now create a smart playlist called "Do Not Exist" with the conditions: Playlist is not "Exist", and Media Kind is Music.


The smart playlist called "Do Not Exist" now contains all the audio listings that do not exist on disk. To remove these listings, select all of them and hold down Shift as you press Delete.