Originally only applied to government agencies, these requirements have expanded to include most other entities. The most relevant line from the section508.gov website, regarding accessibility, is "Under Section 508, agencies must give disabled employees and members of the public access to information comparable to the access available to others". This requirement includes comparable access to websites, electronic documents and videos.
In short, instructional content delivered via video must include a written transcript - either via an attached transcript or by use of closed captioning.