full query

// WP_Query arguments
$args = array(
	'post_type'              => array( 'page' ),
	'post_status'            => array( 'publish' ),
	'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',
		),
	),
);

// The Query
$query = new WP_Query( $args );