Sorry for the overwhelming amount of text, but this page is really for you to zero in on some sources to explore. Listing the options was the most economical way to highlight.
Many of the repositories have all media types, while others are medium-specific.
There are also two tools highlighted for you to either create or adapt existing OER. It's easier than you think!
Figure out where you would like to start e.g., an open textbook for your course, images to complement a presentation, etc. and explore a few resources appropriate for your needs.