Adding complex meta support to WPGraphQL. See: https://wp-graphql.slack.com/archives/C3NM1M291/p1541709817254200

audio-tracks.php

add_action( 'init', function () {
	register_post_type( 'app_audio_track', [
		'label'               => 'App Audio Track',
		'public'              => true,
		'show_in_graphql'     => true,
		'graphql_single_name' => 'AppAudioTrack',
		'graphql_plural_name' => 'AppAudioTracks',
	] );
} );

add_action( 'graphql_register_types', function () {

	register_graphql_enum_type( 'TrackTypeEnum', [
		'description' => __( 'The type of track', 'your-textdomain' ),
		'values'      => [
			'KIDS'   => [
				'value' => 'kids',
			],
			'ADULTS' => [
				'value' => 'adults',
			],
			'VISUAL' => [
				'value' => 'visual',
			],
		],
	] );

	register_graphql_object_type( 'MP3', [
		'description' => __( 'An MP3 file', 'your-textdomain' ),
		'fields'      => [
			'language'    => [
				'type'        => 'String',
				'description' => __( 'The language of the video', 'your-textdomain' ),
			],
			'title'       => [
				'type'        => 'String',
				'description' => __( 'your description...', 'your-textdomain' ),
			],
			'transcript'  => [
				'type'        => 'String',
				'description' => __( 'your description...', 'your-textdomain' ),
			],
			'description' => [
				'type'        => 'String',
				'description' => __( 'your description...', 'your-textdomain' ),
			],
			'audioFile'   => [
				'type'        => 'Int',
				'description' => __( 'your description...', 'your-textdomain' ),
				'resolve'     => function ( $video ) {
					return ! empty( $video['audio_file'] ) ? absint( $video['audio_file'] ) : null;
				}
			],
			'duration'    => [
				'type'        => 'String',
				'description' => __( 'your description...', 'your-textdomain' ),
			],
		],
	] );

	register_graphql_field( 'AppAudioTrack', 'tracks', [
		'type'        => [
			'list_of' => 'MP3',
		],
		'description' => __( 'A List of Tracks by category', 'your-textdomain' ),
		'args'        => [
			'trackType' => [
				'type'        => 'TrackTypeEnum',
				'description' => __( 'The type of track to filter', 'your-textdomain' ),
			]
		],
		'resolve'     => function ( $audio_track_post, $args ) {

			// default to kids if no argument is set
			$type = isset( $args['type'] ) ? $args['type'] : 'kids';

			// replace this with:
			// $meta = get_post_meta( $audio_track_post->ID, 'audio_object', true );
			$meta = [
				'kids'   => [
					[
						'language'   => 'english',
						'title'      => 'kids english title',
						'transcript' => 'kids english transcript',
						'audio_file' => 123,
						'duration'   => '31:58'
					],
					[
						'language'   => 'english',
						'title'      => 'kids english title',
						'transcript' => 'kids english transcript',
						'audio_file' => 456,
						'duration'   => '31:58'
					],
					[
						'language'   => 'english',
						'title'      => 'kids english title',
						'transcript' => 'kids english transcript',
						'audio_file' => 789,
						'duration'   => '31:58'
					]
				],
				'adults' => [
					[
						'language'   => 'english',
						'title'      => 'adult english title',
						'transcript' => 'adult english transcript',
						'audio_file' => 123,
						'duration'   => '31:58'
					],
					[
						'language'   => 'english',
						'title'      => 'adult english title',
						'transcript' => 'adult english transcript',
						'audio_file' => 456,
						'duration'   => '31:58'
					],
					[
						'language'   => 'english',
						'title'      => 'adult english title',
						'transcript' => 'adult english transcript',
						'audio_file' => 789,
						'duration'   => '31:58'
					]
				],
				'visual' => [
					[
						'language'   => 'english',
						'title'      => 'visual english title',
						'transcript' => 'visual english transcript',
						'audio_file' => 123,
						'duration'   => '31:58'
					],
					[
						'language'   => 'english',
						'title'      => 'visual english title',
						'transcript' => 'visual english transcript',
						'audio_file' => 456,
						'duration'   => '31:58'
					],
					[
						'language'   => 'english',
						'title'      => 'visual english title',
						'transcript' => 'visual english transcript',
						'audio_file' => 789,
						'duration'   => '31:58'
					],
				],
			];

			// Return the type of tracks based on the argument
			return ! empty( $meta[ $type ] ) && is_array( $meta[ $type ] ) ? $meta[ $type ] : null;

		}
	] );
} );