Here is an open source accessible audio/video player that you can use to ensure your multimedia presentations are accessible, but just using this player does not make your presentation fully accessible, adding closed captioning, text-based audio descriptions, transcripts, signlanguage track, etc. must also be included.
When an image, table, graph, etc. needs an extended description, doing this in a way that is accessible and reduces the cognative load on the reader is very important. Here are some best practices on how to do this for both the web and EPUB digital ebooks.
Update: Jan 2, 2018
The DIAGRAM Center is a Benetech initiative supported by the U.S. Department of Education, Office of Special Education Programs (Cooperative Agreement #H327B100001). Opinions expressed herein are those of the authors and do not necessarily represent the position of the U.S. Department of Education.