Try this
var $altContent = $elem.children('img').attr('alt');
alert($altContent);
$('.fp_preview').prepend(
'<div id="video-flv">'+
' <object classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000" codebase="http://fpdownload.macromedia.com/pub/shockwave/cabs/flash/swflash.cab" width="720" height="480" id="flv">'+
' <param name="movie" value="WNFlvPlayer.swf" />'+
' <param name="allowfullscreen" value="true" />'+
' <param name="flashvars" value="file='+$altContent+'&autostart=true" />'+
' <embed src="WNFlvPlayer.swf" flashvars="file='+$altContent+'&autostart=true" allowfullscreen="true" width="720" height="480" name="flv" type="application/x-shockwave-flash" pluginspage="http://www.adobe.com/go/getflashplayer" />'+
' </object>'+
'</div>'
).fadeIn(2000);