query AudioBooks

$meta_query = array(
	'relation' => 'OR',
	array(
		'key'     => 'amazon_us_audio',
		'compare' => 'EXISTS',
	),
	array(
		'key'     => 'amazon_uk_audio',
		'compare' => 'EXISTS',
	),
	array(
		'key'     => 'audible_us',
		'compare' => 'EXISTS',
	),
	array(
		'key'     => 'audible_uk',
		'compare' => 'EXISTS',
	),
	array(
		'key'     => 'itunes',
		'compare' => 'EXISTS',
	),
);