So, the footage is actually missing rather than being there but just not playable. Do you by chance have a transition at the end of the clip? If so, remove it. Also, there are fade drag-handles at the upper corners of the blue audio part of the clip. Make sure that they are moved all the way to the outside edges of the clip.
If no luck there, try putting the clip into a newly created project and see if that cures the problem of the song being cut off.
If still no luck, try deleting preferences. To delete preferences open iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. It is a safe procedure. You may need to reboot to have have the procedure take effect.
-- Rich