HELP! Audio disappearing in timeline!

[Peter Reynolds]

I'm editing a project in CS3

I mistakenly did my editing as an interlaced project (with progressive footage), so I created a
new progressive project and have been copying the sequences over (there are 13).

I can edit them and export as Windows Media. Then I save and shut the program down

When I restart the program however, the audio has disappeared from the timeline. It's still represented on the track, but the levels are just a straight line.

I'm totally confused as to why this would happen. It's also not totally consistent. Happening with some sequences and not others

HELP!

 

[Peter Reynolds]

Follow-up

I right clicked on the audio clip and chose REVEAL IN PROJECT
I then right clicked again and chose REPLACE WITH CLIP / FROM BIN

This seems to have worked, however I would still like to know why this is happening.

Thanks.