audio "missing" in timeline

I have a project that I have already created several DVDs from and they have all worked flawlessly. It is 5 different shows that are being put onto one DVD. As i said, I've already made DVDs from this project with no problems; however, the client now wants to add some chapter points. When I opened the project, 3 out of the 5 shows no longer play the audio in the timeline or in the preview window. The audio is still in the timeline, the original audio file is still in the project window and even when I play the original audio (either in Encore or even windows media) it plays fine. The file doesn't seem to be corrupt or missing, so I'm confused. Any helpful hints or suggestions?