Código ActionScript :
stop(); preload._width = 0; preload._visible = preload_bg._visible = true; preload.onEnterFrame = function(){ tamano_total = _root.getBytesTotal(); if( tamano_total == 0 ) tamano_total = 1; porcentaje = Math.round( _root.getBytesLoaded() / tamano_total * 100 ); txt = porcentaje + " %"; preload._width = porcentaje * 3; if( porcentaje == 100 ){ txt = ""; preload._visible = preload_bg._visible = false; _root.play(); delete this.onEnterFrame; } }
http://www.catedramex.com.ar/2010/07/como-hacer-un-preload-en-flash/