No Sound on iMovie project

So it has happened again! This time on part 2 nearly finished - why does it always happen to the end after 3 weeks work?! (see my cry for help 2 Sept 2013).

I am working on a project with video clips, still photos, music and voicecovers. It is almost finished and today the iMovie suddenly closed down - happens too often. When I logged in again there is no sound on either music or some voicecovers, some voicecovers work. But if i change anything, like making a new voicecover, there is no sound.

I have made a duplicate but it carries along the fault whatever it is.

What can I do to get the sound back? The last time I could more or less pinpoint the section, took out the videoclips, restarted. When putting back the video clips and adding a soundtrack it worked fine.

But this time I cannot pinpoint a section.

It is so infuriating after all that work and time spent on this project.

Is there anyone who can help me with this problem? And I am not a computer wizard!

Thanks very much.

THL