Legacy Audio Shortcode

If your only issue is that the format is incorrect then switch it in the_content hook with the correct version.

---

Non-local content:

Place in a plugin or functions.php

// hook earlier than 10

add_filter('the_content', 'wpse_20160110_the_content_fix_audio', 0);

function wpse_20160110_the_content_fix_audio($content){
    return str_replace ( '[audio:', '[audio src="https://wordpress.stackexchange.com/questions/214052/, $content );
}