Sound and video have become almost indispensable to many applications these days. Sound is most often used as a feedback mechanism, indicating to a user that they can or have clicked on something, providing users with instructions on completing a task, or letting users know they've selected right or wrong answers in e-learning applications. Video is also used for similar purposes, although on the web you might see it used for everything from entertainment to videoconferencing (all of which can be done with Flash). Together, sound and video can take your applications to the next level, not only enhancing the end user experience but also providing information that otherwise couldn't be presented.
Adding sound can be extremely useful if your site caters to people who are visually impaired. It can be valuable to add audio cues to a SWF file, prompting key press actions or reading text aloud. Or, you can create an MP3 sampler for a freelance musician or add a company's jingle when its website loads. Adding tacky or annoying music is fairly easy to do, but detracts from the site. Getting your peers to test your site and provide feedback can help you avoid useless clutter and find subtle, creative, or diverse ways to use sound and video in your SWF files. Because sound and video add a lot of file size to a document, you have to make sure they're worth your bandwidth and the bandwidth of your visitor.