Had to add the "ReadDTD" -> False"ReadDTD" -> False option to: $xml = Import[ "/Volumes/WDC3TBRAID/Downloads/iTunes Music Library.xml", {"XML", "XMLObject"}, "ReadDTD" -> False]
$xml = Import[ "/Volumes/WDC3TBRAID/Downloads/iTunes Music Library.xml", {"XML", "XMLObject"}, "ReadDTD" -> False] to get WReach's solution to work with Mathematica 11.0. Rest of his solution works fine after that.