It's easier to create a folder named Media (and all needed sub folders) in the main game folder. Open the config.xml and copy " <MediaPath Path="Media" /> " before " <MediaPath Path="Media.zip" /> ".
Now you can copy files from media.zip to the media folder (it must be the same path as in media.zip!) and the game should load the tweaked files from the media.
Multiplayer and achievements will get disabled tho!
But you just have to temporary rename the media folder and you can play im MP.