Videos uploaded in the Videos module require that the client has Silverlight installed. This is the same as running flash on a website - you need the browser plugin to view Silverlight content.
As for flash video - the Flash Manager of the rich text editor can only embed flash content which does not require third party players. It can use only flash content that can be rendered directly by the browser - SWF for example. FLV files require third party player component. If you which to use FLV files you can follow the example here
and create a custom field control which will output a FLV file from the Documents & Files module.
the Telerik team
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