Sound lags behind video

Hi guys,

I used Easy Youtube Video Downloader and downloaded a few music videos off of youtube, and I've noticed that the sound lags behind the video by a fraction of a second. The videos are 1080p and in the mp4 format, and the video playback is pretty smooth. However, while the sound is definitely in sync on the youtube page, it lags behind a little bit when I play the downloaded clip using WMP11 (it plays very choppy and even more out of sync if I use Quicktime Player).

What is going on?

Thanks!