If a site contains audio or video clips, you should provide text transcriptions or descriptions of them. Exceptions include clips that merely visualize something already explained in the text and audio clips containing page content in spoken form.
For example, if you have a video clip containing an interview, you should try to create a textual alternative that contains the spoken content. You may need to add notes that describe what happens visually, if it is relevant to understanding.
This often means considerable work, perhaps comparable to the effort of creating the primary content. The cost may thus be too high. In that case, you should describe the situation to users. People should at least know that there is essential content that they cannot access, and there should be some note about the nature of the content. If a page contains a link to the clip, the text of the link could and should (cf. to rule 2) indicate what the clip is about.
Of course, there is content for which no adequate text transcription or description can be written. Instrumental music is a typical example.