[Greg Evans]

I am experiencing this right now and have had to deal with this in the past. I previously found a work around to this in a forum somewhere.
From memory it involved creating a clean project preferably using an Easy Setup. Reimport the AIFF or WAV into this clean project.
Create a new timeline with the correct setup to match your old timeline and paste the contents of the old timeline into the new. Then edit in your problematic audio file and check sync.

Hopefully all will be well but I have just tried this again to solve my current issue and it has not worked.
So it is still a mystery as to why it happens and how to rectify consistently.
If I have success I will post my adjusted method.

I am hoping we will not see this problem in FCP 7 but I am not confident.