Add audio attachment link to RSS

You can add this part into your function:

$audios =& get_children('post_type=attachment&post_parent=".$post->ID."&post_mime_type=audio' );
foreach ( $audios as $id => $audio ){ 
    $content.='<a href="'.wp_get_attachment_url($id).'" target="_blank">'.$audio->post_title.'</a> ';
} 

to add audio links into your feed content.

I’m using get_children() here, you can read more about it here:

http://codex.wordpress.org/Function_Reference/get_children

Edit:

Here is the whole function:

function featuredtoRSS($content) {
    global $post;

    if ( has_post_thumbnail( $post->ID ) ){
        $content="" . get_the_post_thumbnail( $post->ID, 'topImage', array( 'style' =>  'margin:0 auto; border: 1px solid #555; display:block;' ) ) . '' . $content;
    }

    $audios =& get_children( 'post_type=attachment&post_parent=".$post->ID."&post_mime_type=audio' );
    foreach ( $audios as $id => $audio ){ 
        $content.='<a href="'.wp_get_attachment_url($id).'" target="_blank">'.$audio->post_title.'</a> ';
    } 

    return $content;
}

add_filter('the_excerpt_rss', 'featuredtoRSS');
add_filter('the_content_feed', 'featuredtoRSS');