Multimedia presentations can be inaccessible to people who are unable to hear the audio content. The solution is to provide captions, a text version of the audio content that is synchronized with the multimedia presentation.
The technical standards and specifications for captioning video are well established and documented. For additional information relevant to online multimedia, consult the AccessIT Knowledge Base article What is the difference between SMIL and SAMI? 
The standards for the style of captions are less clearly defined, although a variety of guidelines have been developed. The following are a few of the more prominent examples.
The most comprehensive resource is Captioning Key: Guidelines and Preferred Techniques  (in PDF; requires the free Adobe Reader ), developed by the Captioned Media Program  (CMP) at the National Association of the Deaf. This document provides extensive detail and practical examples regarding text style; caption placement; line division; presentation rate; language mechanics, including grammar, punctuation, and quotation marks; and a wide variety of special considerations, such as when and how to caption sound effects, when and how to caption music, how to caption foreign language or dialect, and how to caption numbers.
The WGBH Media Access Group Captioning FAQ  is a public version of WGBH's internal reference manual and covers many of the same issues covered by the CMP guidelines, although in considerably less detail. WGBH has been involved in captioning for over thirty years, and their FAQ draws on this experience.
The National Center on Accessible Media, another WGBH group, credits the guidelines contained within the Captioning FAQ but offers as addenda eight general conventions for captions, as well as six conventions for math notation and four timing conventions. The addenda are contained within Appendix 3  of the document Making Educational Software and Web Sites Accessible: Design Guidelines Including Math and Science Solutions .
As educational entities prepare to caption video content, consulting these references can help ensure consistency and high quality.
-  What is the difference between SMIL and SAMI?
-  Captioning Key: Guidelines and Preferred Techniques
-  Adobe Reader
-  Captioned Media Program
-  WGBH Media Access Group Captioning FAQ
-  Appendix 3
-  Making Educational Software and Web Sites Accessible: Design Guidelines Including Math and Science Solutions