Maybe because it has an itch? :-)
If it does it on only one video, I'd suspect a defect in the video file. If it does it on all sound files including MP-3's, either the speakets or the sound hardware on the computer. If it does it only on video files but on all of them, download and replace the codec for that file type.
Answered Jun 07, 2011
Edited Jun 08, 2011