Register custom meta box on custom post type to show in wp rest api

functions.php

/*
*  Register custom meta box on custom post type to show in wp rest api
*  
*  Custom post type must support 'custom-fields' and have the 'show_in_rest' parameter set to true 
*/

//Custom post type is 'audio' and custom meta box is 'cloud_url'
add_action( 'rest_api_init', function () {
    register_rest_field( 'audio', 'cloud_url', array(
        'get_callback' => function( $post_arr ) {
            return get_post_meta( $post_arr['id'], 'cloud_url', true );
        },
    ) );
} );