How to adjust audio latency?

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My application involves voice narration and a video feed of the webcam, inside a small mplayer window on-screen to be captured along with everything else. The problem is that the webcam previewer has a bit of lag, causing the audio to play about 0.5s ahead of the video.

Is there a easily known way to tell ffmpeg to add latency to the audio? I would rather not do this with post-processing.