Apple Final Cut Express 4 - Manual (Page 431)

Chapter 30 Linking and Editing Video and Audio in Sync 431
VI
An out-of-sync indicator appears whenever the following conditions occur:
 Audio and video clip items from the same media file are out of sync. Because they
come from the same media file, these items always show out-of-sync indicators, even
if they are not currently linked.
 Audio and video clip items have been linked together in the Timeline, and then
moved out of sync.
Audio and video clip items that have a sync relationship must be vertically overlapping
in the Timeline for out-of-sync indicators to appear when the items are moved out of
sync. No indicator appears if linked audio and video clip items are so far apart that they
no longer overlap.
If you move these items back together so that they overlap, the indicators appear again.
This also works with multiple instances of clips from the same media file on disk. For
example, suppose you have three items in your sequence, all from different parts of the
same media file.
Audio moved away
from video clip
Audio moved back
under video clip