I noticed that after I got my MacBook Pro that the Megavideo's audio was out of sync, sometimes up to 5-10 seconds. Sometimes, when the lag is only about a couple of seconds you can let it slip by not focusing on an actors lips, but when the lag is greater than a couple of seconds it makes it impossible.
Usually this is due to a codec, on a windows machine it's quite simple, download the latest version of K-Lite codec pack and 9 times out of 10 the problem is solved. If that didn't work then you can get a codec detection program and try and hunt...
