wp_audio_shortcode displays 'array'

Not sure if this belongs in the Plugins forum, but…

I’ve set up a custom post type using the pods framework and created a file upload field. When I try to output in media player on the front end with the code below the word ‘array’ displays. What am I missing?

<?php
    $audio = get_post_meta($post->ID, 'audio', true);
    $attr = array(
        'src'      => $audio,
        'loop'     => '',
        'autoplay' => '',
        'preload' => 'none'
      );
      echo wp_audio_shortcode( $attr );
?>

Thanks for any help