In both instances, when you use the up and down arrows it will adjust your audio by 1/10th of a second per click.

So in reverse, if the audio is behind the video you’ll want to use the “down arrow”. That will ‘delay’ the audio, allowing the video to ‘catch up’. That means if the people in the video are talking and you can hear them before their lips move, you’ll want to use the up arrow. Use the “up arrow” if the audio is ahead of the video.Locate the section titled Audio/Video – within that window there’s a ‘timer’ with “up and down arrows” to adjust the timing of the audio.Open the video with the out of sync audio in VLC and then select Tools -> Track Synchronization (Mac users: select Window -> Track Synchronization).Note: this will work for all video formats, not just mkv/mp4/avi – they’re just common formats (and the 3 that I’ve found most often have sync issues).

If you aren’t using VLC yet, head over to their download page and give it an install. The quick solution is to use the amazing (and free) media player – VLC, and its easy to use built in audio delay/forward feature. Temporarily Fix Out of Sync Audio and Video

There are two ways to fix out of sync audio and video – one is temporary and quick, the other is permanent and a bit more involved, although still quite easy. This step by step guide will show you how to fix out of sync audio and video in a video file.

Avidemux instructions