I don't have a brief answer for you, but I would highly recommend this video and the associated guide from Doug Harriman (University of California): Achieving Optimal Accessibility in Storyline and Other eLearning Environments (YouTube, 48 minutes).
The video is broken into multiple chapters and includes a link to the latest version of the UC Storyline accessibility checklist. See pages 27-33 of the checklist for a detailed guide to using buttons, layers and triggers for media, but you should review the whole document in context so that you have a cohesive approach to an accessible course. The video and the checklist provide an excellent introduction to Storyline accessibility, including best practices, lists of known accessibility glitches, and many truly useful tips.
