Being able to play audio and video files, often years after they were originally created, has been a steadily growing problem for audio, video, and computer forensic examiners.
Of course, video examiners encounter this almost daily if they are dealing with security digital video recorder (DVR) files because of the proprietary codecs (which is short for COder DECoder, and is the software program that converts video to and from a particular digital file format) used by manufacturers of the systems to lock users into their brands.
Even for those who do not process security DVR every day, things have been getting worse. Part of this is due to changes in the latest Windows Media Player where Microsoft removed one of the popular codecs previously used to encode/decode AVI files. Many people are under the impression that AVI is uncompressed and either doesn't need a codec or only has one universal codec. This is far from the truth. AVI, like WAV, is a very flexible format and it supports the use of almost any audio or video codec. To play the file, you have to have the correct codec - period. No codec, no playback - it is that simple.
I was faced with this problem just recently due to the Microsoft changes. The files were uncompressed AVI and would not play on three different machines, even though they played back correctly only two months before. A bit of digging found the cause. The solution ended up being to download an audio/video codec pack. I used the free one found here, but there are others out on the Internet also.
You can imagine how this can be a problem for law enforcement where evidence may need to be archived for up to 25 years and still be able to played back. All it takes is one automatic update and the ability to playback can cease. This is why I recommend to my students that they always include a copy of a freeware player that will play back the media as part of the archived evidence.