If you are experiencing this while viewing the captured video in the SnagIt editor, try rendering the video out to a finished format such as ".mp4" and view the footage in WMP or some other media player. It is likely the video and audio will be synced up correctly in the rendered video.
I too ran into this issue and spent over 2 hours trying to figure it out to no avail. I ended up just going with Debut Video Capture by NCH software. While the video quality is not quite as sharp, the sound and video sync just fine.