I doubt it converted them. It probably just assigned itself as the default player for those extensions.
You should be able to go back into folder options and reset the default program for those file types. If the extension really is changed, try to set it back to the original and open it with the correct player. Again, I don’t think even a program as malicious as media player could have done a true conversion of the files.
Everything that Waverly said.
Also you can run windows media player and click on Tools>Options and select the “File Types” tab.
Uncheck all the file types you dont want windows media player to open.This should let the original players to play the software.Just in case you still cant get the files to open with the player you want all you have to do is right click on the file and choose “open with” and click “choose program”.Select your wanted program from the list and dont forget to check “always use this program to open this”
You should also go under Windows Explorer, select View>Options, and uncheck “hide extensions for known file types.” This option causes more problems than it solves once the user passes the “totally clueless newbie” stage.