audio disappearing from imovie 09

Hi, I'm a beginner at this. Hope someone can help.

I've started importing Hi8 analogue tapes via a Sony Digital 8 camcorder onto a MacBook with iMovie 09. I'm storing them straight onto an external hard drive.

On several of the event files, the import works fine. But recently, the sound has been disappearing after import. The sound is there on imovie after import, but on coming back to some files later I find it has gone. It is still there on the smaller imovie files (10 hours of taping and less). Could the size of the file be the issue?

I've tried restarting. The Sound settings on System Preferences seems ok. Any ideas?

Tech info: Macbook is OS X, version 10.6.6, 4 GB. Sony digital 8 is TRV-110E. External Hard drive is WD 1 TB, connected by Firewire 800. The clips are saved in the Hard drive as .dv files.

If you move your audio file from the original place that you imported it from...iMovie will no longer recognize the file. Replace the audio...keep the file where it is until you export the movie...then do what you want with the audio file.

Thanks for taking the time to reply Codda. But I don't follow. I'm very new to this!
All I've done so far is import analogue video + sound combined straight into imovie. I've got 7 separate files/clips that have been created so far. Sound plays back ok on 4 but no longer on 3. It was there on those 3 earlier though, after import. I've done nothing yet besides importing - no editing yet.

The following may be relevant? Under Edit, "Detach audio" is greyed out; "Mute Clip" is off. Under View, "Audio Skimming" is ticked.

What I was referring to was....when you imported the original files...do not then move from that location. Drag your audio file back in...and leave the original file alone until your done.