In this video, Margaret Stewart talks about Youtube system to manage with uploaded videos and copyrights.
I've found this quite interesting; it shows how different media companies decide different policies as owners of songs, video etc...
That's why some videos are removed and some others have the suggestion to buy a certain (background or not) song from ITunes, for example.
Moreover, it made me think about the check complexity of that system, as it has to deal with a huge amount of files at the same time.
UPDATE: as the html tag seems to have failed, this is the link http://www.ted.com/talks/margaret_stewart_how_youtube_thinks_about_copyright.html