the existing code threw a type conversion error for me
//var responseVariables:URLVariables = URLVariables(event.data);
I changed it to use the decode method and it worked fine
protected function getRequestTokenCompleteHandler(event:SoundcloudEvent):void
var responseVariables:URLVariables=new URLVariables();
var str:String=new String(event.data);
responseVariables.decode(str)
requestToken=createTokenFromURLVariables(responseVariables);