This happened using Audio Sound Designer. The audio went out of sync at one hour in a 1 hour and 45 minute documentary. I'm posting this for a non-English speaker who is stumped by this. Let me know if more information is needed.
The sound sync issue may be the dreaded FCP feature of stamping a frame rate on imported audio which is wrong. Matt Lyons tutorial explains why this happens and how to correct this.Also read his comments as he has discovered some new techniques.