Embedding media resources within an EPUB ensures that a future reader will be able to locate these resources and view them in the original context of the work. In order to keep the overall size on an EPUB manageable for access, it may be advantageous to embed lower quality copies of the media and link to higher resolution versions via persistent links such as DOIs.
These guidelines also refer to where media content is hosted:
14. Avoid depending on externally hosted web services in general
29. Consider a preservation-specific version of the EPUB