Hi Yestyn. What video editing program are you using. There are several programs I use which allow for fairly easy "syncing" of the sound. It would also be good to know what kind of recording equipment you are using. I use a Canon 7D which has built in audio fed by my Zoom H4N. While this is recording audio on the CF card in the camera, my audio is also recording directly in the recorder on an SD card. This means I get one audio/video output and a second audio output. The pure output from the H4N is high quality stereo output which is always better than a camera.
I then import the video with audio to one editing track (I use Final Cut Pro on my Apple and also Pinnacle Studio HD on my PC).
I then separate the audio channel out of the video by muting it. I then copy in my audio from the H4N and insert it directly below the muted audio so I can then edit the audio, add effects if you wants, etc.
The key thing is do NOT edit the audio and take out parts on the video portion as that is what throws off the sync. Thus, match up the audio, then cut the video and audio together. If you have an editing window open for both audio tracks you can see the histogram or graphic of the audio peaks. The will match up perfectly and both applications have an ability to see those matching peaks and align them when you get close.
You can also manually slide the audio a bit forward and backward in Imovie but it's not as sophisticated as the other applications.
If the sound is ahead of the video, you can slide your video track forward and slide the audio back, or just start clean with the full audio and video. Did you edit things before trying to align up the sound? That's usually what happens. Let me know how it goes or what your using and I can try to help you more.
If you use a marker right when you start (you know like in the movies when they say "TAKE ONE" and clap that board...that's actually a tool used to mark the audio for later syncing ;-) This lets you align the exact spots...it's really simple, just clap your hands loud and you'll get your marker that you'll easily be able to see on both tracks.
Here is one tutorial:
http://www.youtube.com/watch?v=GPCXPxTyZbM
Cheers!
