Sound files, QuickTime, Flash, PDF, Java applets... anything that requres a player or plugin may require accomodation.
A sound file, such as an audio recording (no video) of an interview, is a non-text element that must be accompanied by a transcript for people who can't hear.
A multimedia presentation, which has both audio and video, must have captions for the audio and those captions must be synchronized with the video. The captions will serve not only people with disabilities, but people in noisy environments, like airports, or in quiet environments where they can't listen to the sound.
For people who can't see, you must provide synchronized audio description. That is, you need running commentary describing anything happening in the video that isn't apparent from the audio.
Example of synchronized captions and audio description. (To view this example, you will need QuickTime Player, a free download.)
Whenever a plugin like QuickTime Player, Flash Player or Adobe Acrobat Reader is required, you must provide a link to download the plugin.
Many Flash, Java and PDF files can be made accessible, but often they are not.