So the video is just matched to the audio from like the CD? If you are cutting a performance that they are lip syncing to I would say it probably does go out a bit every now and then.
If so is the client saying the it is going in and out of sync? It Drifts more out of sync as it plays? It is out of sync at the same rate throughout the video?
How good are you at telling if things are in sync? Have you compared the FCP timeline to the encoded piece?
What codec did you encode it to? what type of system, how fast is the client’s?