Append a Download link to the WordPress audio shortcode output

download-link-to-audio-player.php

<?php
/*
Plugin Name: Append Download button below audio player
Plugin URI: https://www.damiencarbery.com/2017/02/add-download-link-to-wordpress-audio-player/
GitHub Plugin URI: https://github.com/damiencarbery/audio-shortcode-download-link
Description: Add a Download link to the html generated for the "audio" shortcode.
Author: Damien Carbery
Version: 0.2
*/


if ( ! defined( 'WPINC' ) ) {
	die;
}


add_filter( 'wp_audio_shortcode', 'ast_audio_shortcode_enhancer', 10, 5 );
function ast_audio_shortcode_enhancer( $html, $atts, $audio, $post_id, $library ) {
	/*error_log( 'HTML: ' . $html );
	error_log( 'ATTS: ' . var_export( $atts, true ) );
	error_log( 'AUDIO: ' . var_export( $audio, true ) );
	error_log( 'POST_ID: ' . var_export( $post_id, true ) );
	error_log( 'LIBRARY: ' . var_export( $library, true ) );*/

    $audio_types = array( 'mp3', 'ogg', 'wma', 'm4a', 'wav' );

    // Use the first audio type that has data.
    foreach ( $audio_types as $extension ) {
        if ( array_key_exists( $extension, $atts ) && strlen( $atts[ $extension ] ) ) {
            return $html . sprintf( '<p><button type="button"><a href="%s" download>Download</a></button></p>', $atts[ $extension ] );
            break;
        }
    }
	
	// For an externally hosted file the [embed] shortcode is active and the url will be in $atts['src'].
	if ( array_key_exists( 'src', $atts ) ) {
		return $html . sprintf( '<p><button type="button"><a href="%s" download>Download</a></button></p>', $atts[ 'src' ] );
	}

    // Otherwise return the original html.
    return $html;
}


// Add a Download button below the Audio block.
add_filter( 'render_block', 'dcwd_add_download_button_to_audio', 10, 2 );
function dcwd_add_download_button_to_audio( $block_content, $block ) {
	if ( 'core/audio' == $block['blockName'] ) {
		if ( preg_match( '/src="(.*)"/', $block['innerHTML'], $matches ) ) {
			return $block['innerHTML'] . sprintf( '<p><button type="button"><a href="%s" download>Download</a></button></p>', $matches[1] );
		}
	}

	return $block_content;
}

wp_audio_shortcode-filter-data.txt

html:'<!--[if lt IE 9]><script>document.createElement(\'audio\');</script><![endif]-->
<audio class="wp-audio-shortcode" id="audio-1492-1" preload="none" style="width: 100%;" controls="controls"><source type="audio/mpeg" src="http://example.com/wp-content/uploads/2017/02/audio-track.mp3?_=1" /><a href="http://example.com/wp-content/uploads/2017/02/audio-track.mp3">http://example.com/wp-content/uploads/2017/02/audio-track.mp3</a></audio>'
atts:array (
  'src' => '',
  'loop' => '',
  'autoplay' => '',
  'preload' => 'none',
  'class' => 'wp-audio-shortcode',
  'style' => 'width: 100%;',
  'mp3' => 'http://example.com/wp-content/uploads/2017/02/audio-track.mp3',
  'ogg' => '',
  'wma' => '',
  'm4a' => '',
  'wav' => '',
)
audio:NULL
post_id:1492
library:'mediaelement'

wp_audio_shortcode-filter-test.php

<?php
add_filter( 'wp_audio_shortcode', 'ast_audio_shortcode_filter_test', 10, 5 );
function ast_audio_shortcode_filter_test( $html, $atts, $audio, $post_id, $library ) {
    error_log( "html:".var_export( $html, true )."\natts:".var_export( $atts, true )."\naudio:".var_export( $audio, true )."\npost_id:".var_export( $post_id, true )."\nlibrary:".var_export( $library, true )."\n" );
    return $html;
}

Thanks for the combined effort peoples. Worked for me. At least until some major release of WordPress changes things again. And just how secure is it against damned invaders? No idea what to do with Shortcode filters. I liked the CSS of @meghansmith but I have no clue how to implement those filters she posted on her profile page. But @zizoua5000 gave me enough to work with. Thank you.