Media filters are very wrong...

[Denis-de-Bernardy]

e.g.:

apply_filters($callback, call_user_func($callback));

...

$html = apply_filters('audio_send_to_editor_url', $html, $href, $title);

so, basically, we get to add fields over on the type_url_form_audio form, but they're not available as filters in the audio_send_to_editor_url filter.