Sound on video not syncing with picture

I've recently downloaded some music. Some of the items were videos. At first, the longer videos all returned an 8387 error message. I managed to resolve that.

The songs are fine, but the videos are not. They can be seen (the picture) but the sound is either delayed or missing (almost like a buffering problem). This wasn't the case yesterday when I downloaded the content, so why all of a sudden? I've made sure I have the latest version of ITunes. Please help?