lost sound

I'm on a big project. Imported 24 hours of video. I'm ten hours into editing, pulling about one hour of clips down to the timeline at a time. I pull down the next one hour clip and there is no sound. I check the earlier ten hours of clips that I have just edited over the past week and there is no sound there either. I save my work, shut down iMovie and restart, and everythings fine - the sound returns.
I continue to work, same thing happens again, only this time, the sound doesn't return. Help.