That sounds (mysteriously) like the infamous drop/non-drop problem, or the difference between 23.98 and 24.00. What speed is your video material? What speed is your sound? What specific formats are you using?
What happens if you render out to ProRes 422HQ and then later convert that to H.264 (assuming that's where you're going with this)?
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At the moment, I am reediting a video which I made a couple of weeks ago, and which was rendered using the standard H.264 Master settings. The original footage was shot with a Canon 60D.
I tried out footage from different cameras, and all have the same problem, they are all off by 7 frames.