Looks like WordPress changed the post metadata format but didn’t update the rss_enclosure function to work with the metadata update. There’s no longer an enclosure key in the metadata array, but the enclosure is already generated in the metadata array with the key “mp3_enclosure”.
Instead of using rss_enclosure(), something like this will output the MP3 enclosure:
$metadata = get_post_custom($post_id);
echo $metadata[‘mp3_enclosure’][0];