There is no way to set "alternate" text for flash content when you are adding it with the RadEditor dialogs. I think that making a flash movie accessible is not as simple as providing an "alt" attribute. The movie itself should be created with accessibility guidelines in mind.
The <embed> tag is not part of the XHTML specification, but since the non IE browsers cannot play OBJECT tags (like Firefox and Chrome), the Flash and Media managers insert it to provide cross-browser support for Flash and Video files.
If you would like you can modify the output from the Media and Flash managers in the OnClientPasteHtml event of RadEditor.
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