By default, reading a .FLV file from another flash-file doesn’t work because the MIME-type of the .FLV file is missing. You need to explicitly define this (ie: in IIS) to allow these files to be interpreted as flash-files.
You can do so by logging into the IIS Manager, and right-clicking the particular website. Select Properties.
At the tab “HTTP Headers", at the bottom, there is an option to define “File Types". Add a new MIME type, and define it as “flv” extension and “video/x-flv” as content type.
Save it, and you’re good to go.