Educational content that is presented in audio is inaccessible to students who are deaf and to some who are hard of hearing. If the content consists of audio only, a printed transcript is an accessible alternative. However, if the content includes both audio and visual components, captioning is preferable because it allows the video and text equivalent to be synchronized, thereby giving a hearing-impaired participant access to content equivalent to that provided to a participant who can hear the audio.
The World Wide Web Consortium's Web Content Accessibility Guidelines and the Access Board's Section 508 Standards both support the idea that equivalent alternatives to presentations that include video and audio must be synchronized with the presentation. For more information about these guidelines and standards, see the AccessIT Knowledge Base article What is the difference between the W3C® guidelines and the Section 508 standards for web accessibility? 
-  What is the difference between the W3C® guidelines and the Section 508 standards for web accessibility?