In your VIEW drop down menu, go down to the bottom and select Audio Playback. Then click AUDIO FOLLOWS VIDEO. I think that will solve your issue that is if your using FCP 5 or above. I had this problem a few weeks ago too. What I didnt like though is that when I switched it to audio follows video I was not able to monitor the audio via my Studio Monitor speakers. The audio was coming from my external monitor source. So I switched it back so I can hear all audio and sounds like I am used to hearing them. It will be in sync on output though.