Retrieve ONLY mp3s and set a variable for use in a playlist called by single.php template?

So here’s what I did to resolve this…

I inserted the following code right after the loop started:

<?php 
    global $post;
    if( is_singular() && is_main_query()  && $post->post_type === 'download' ) {
        $download = new EDD_Download( $post->ID );
        $files = $download->get_files( );
        $submission_attachment_ids="";
        if ( is_array( $files ) ){
            $first = true;
            foreach ( $files as $key => $value ) {
                if ( !isset( $value[ 'attachment_id' ] ) ){
                    continue;
                }
                if ( $first ){
                    $submission_attachment_ids .= $value[ 'attachment_id' ];
                    $first = false;
                } else {
                    $submission_attachment_ids .= ',' . $value[ 'attachment_id' ];
                }
            }
        } else { 
            die; // no files attached to the download
        }    

        if ( !$submission_attachment_ids || strlen( $submission_attachment_ids ) < 1 ) {
            return die; // no files attached to the download
        };

?>

Then, just above the content block I inserted my player call, which drops it right under the header:

<div id="player" width="100%">  
    <?php echo do_shortcode('');?>
    </div>