The publisher of Flash, Adobe, has an official white paper that discusses accessibility in the context of Flash animations. You can find this document at http://www.adobe.com/resources/accessibility/best_practices/best_practices_acc_flash.pdf.
The first item of the “best practices” is to provide text equivalents. This can be done by using the accessibility panel of Flash. This permits visually impaired users understand the content of an animation or movie.
Note that Flash also allows the design of interactive control elements. Many of these visual control elements are inaccessible to some users. However, it is possible to create completely hidden control elements that are not displayed but presented by screen readers.
Note that many options to enable accessibility are only accessed via ActionScript. This means it takes more expertise to make a Flash animation accessible (compared to the creation of the original visual content).