Did You change or move anything regarding iTunes - or CDs where audio was from.
iMovie Project is NO MOVIE at all as one might think - it is just a small text document pointing to the material You want in Your movie and How it is supposed to be played to play as Your Movie.
Compare to a Symphony Orchestra and the Notes the Conductor use - They will never play without the orchestra (or MIDI-samples)
So if You for example use audio from an Audio-CD - it will play nicely in iMovie - but when CD is ejected it all goes silent.
This is the way NON-Destructive Video editing programs works - it's no BUG or fault.
Yours Bengt W