If you do not control the copyright on the material you are displaying, you need to be aware that Fair Use does not necessarily grant permission to use an item however you wish.
For a good overview of copyright, go here.
If you are embedding a video from an outside source such as YouTube, you may find at some future time that the Copyright Owner has issued a takedown notice thereby making it impossible for you to embed that particular media item.
Copyright friendly resources
Here are a couple sites that can help you find materials for use on your wiki site.
More info on copyright
If you are unsure about your use, or you wish to learn more about the copyright limitations on a particular piece of work, you can learn more from these resources.