It seems like you might have the wrong value in your hidden form element. You are currently setting it to this:
If this is the path to where the file is this is not the correct value to use here. You must set this hidden form element to be the name of the file, you have already set the path to the file inside of the Digital File Pro download server behavior.
You can test out the download server behavior's values by adding this line of code just after line 101:
die("WA_DownloadResult1, " . "../../mp3/, ". "".((isset($_POST["hiddenField"]))?$_POST["hiddenField"]:"") .", " . "[FileName], " . false . false . false . ", ". ", ". ", ". "");
This will let you know the values that are used inside of the download function call, please post back with this info if you are having any more trouble with this.