Custom RSS Feeds & Post Meta Data

Rss feeds normally display media files along with post excerpts. I am using post meta to add media files to a post. Unfortunately, since the audio file is not being inserted into the main textarea the audio files are not being displayed in the RSS feed. How can I edit the RSS feed in such a way that the rss_enclosure() displays media files from post meta?

This is the code I’m using to edit the RSS feed:

function custom_postrss( $content ) {
global $wp_query;
$postid = $wp_query->post->ID;
$audiourl = get_post_meta( $postid, 'custom_audio_url', true );

if( is_feed() ){
    $content = '<div class="media">This post has an audio file. '.$content.'Subscribe for more.'.$audiourl.'</div><!-- .media -->';
}
return $content;
}
add_filter('the_excerpt_rss', 'custom_postrss');
add_filter('the_content', 'custom_postrss');

This only returns the URL, but not the media file box that is usually seen in RSS feeds. Any ideas? I’d be happy for any pointers.