a wild guess. The Audio was on an Audio-CD that now are ejected ?
Put it in again and it will work.
As iMovie doesn't create a MOVIE in Projects but just a text document on what material is to be used and where to find it to Play Your edited version. Moving any of them - Video-Clips, Photos and Audio files - and iMove get's lost. (This is how NON-Destructive Video editing is supposed to work)
So do not move or alter any folder named
• iMovie Event's - or -
• iMovie Project's
or folder containing Audio and Photos used in project. I start by creating a New folder - naming it with Date and Movie name - and in this I put all needed components. And this I stick to till Movie is done and Exported and Never need to be edited any more.
Yours Bengt W