When media player software gets updated, oftentimes (unless the user instructs the installer NOT to make the changes) file associations get shifted to the new program being installed. We have seen this happen most commonly with iTunes, Quicktime, Windows Media Player, and Real Player. Updating the desired media player and then re-associating the file format to that media player usually resolves the issue.
Here is how to re-associated file types in Windows:
http://technet.microsoft.com/en-us/library/cc749986.aspx