As the assistive devices are generally text-based, each image or sound must have an alternative text. Even the image or sound that punctuates, highlights, separates content, or helps control the layout of the page must be represented by text. The alternative text must clearly state why the graphics are in that specific part of the page and how they fit into the continuity of the lineal stream of the text.
Although current browsers support an ALT attribute for image tags, they do not offer such an attribute for the EMBED and OBJECT tags. When providing the viewer with alternate content (if the plug-in content cannot be displayed), other options may be considered.
Techniques for Web Content Accessibility Guidelines 1.0, W3C: Provide equivalent alternatives to auditory and visual content.
Is your chosen medium accessible to people who have disabilities?