например вот так:
HTML:
<object classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=8,0,0,0" width="100%" height="100%" id="index" align="middle">
<param name="allowScriptAccess" value="sameDomain" />
<param name="movie" value="index.swf" />
<param name="quality" value="high" />
<param name="bgcolor" value="#252525" />
<param name="FlashVars" value="myURL=http://flv/video.flv">
<embed src="index.swf" FlashVars="myURL=http://flv/video.flv" quality="high" bgcolor="#252525" width="100%" height="100%" name="index" align="middle" allowScriptAccess="sameDomain" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" />
</object>
во флэше:
var videoPath:String = myURL; // или var videoPath:String = _root.myURL;
player.source = videoPath;