In HTML authoring, there are very good reasons to include an alt attribute into every img element. The purpose is to specify a textual replacement for the image, to be displayed or otherwise used in place of the image. Thus, the prime rule is: Consider what the page looks like or sounds like when images are not shown. Then, write for each image an alt text that best works as a replacement. This document also gives more specific suggestions for simple, common situations, and some uncommon too. For content-rich images, it recommends explicit links to textual alternatives.
Bland presentations come to life when you add audio and video elements. Audio also becomes much more important when you are sending presentations online. Fortunately, Hello Slide is a text-to-speech mashup that lets you add audio overlays to presentations and then host them for you.
W3C Working Draft: author conformance requirements for use of the alt attribute in HTML5 and best practice guidance for authors of HTML documents on providing text alternatives for images