When you reload a previously saved document, some of the photos and audio may be marked as missing. A red marker will appear in the timeline for each missing item (see Dealing with missing media in the timeline).

You can locate missing photos by selecting the photo in the timeline and choosing PhotoReplace Missing Photo.... You will be asked to locate the missing photo. Once you have located one of the missing images, Photo to Movie will attempt to locate any other missing images in the same directory.

You can also locate missing audio by selecting the audio in the timeline and choosing AudioReplace Missing Audio.... You will be asked to locate the missing audio. Once you have located one of the missing audio items, Photo to Movie will attempt to locate any other missing audio items in the same directory.