Thanks for your suggestions.
I know that By Frame is set to 1, so that isnt it.
I'm not compositing CG into video, I have before, and used frames, and it matched fine.
What I'm doing is lip syncing, and the render is faster than the audio. I've spoke to people on my course, and some have the same problem, but none of us know how to fix it.
I just cant think why the playblast would be different, unless it's operating on a different frame rate!?