Dear developer.

Even though the flv_player.swf is shipped in an OOTB distribution of TinyMCE Alfresco has decided NOT to include it.
The reason is because security vulnerabilities have been reported against the flv_player.swf.
If required in your Alfresco installation, for whatever reason, feel free to download the flv_player.swf from
Moxie Code's website and include it, but be aware that you are doing so AT YOUR OWN RISK.

Best regards, The Alfresco team