TIL that there are multiple different published specs for how to provide alternative formats for a podcast (e.g. audio + video). There doesn’t seem to be clear agreement on exactly which tag you should use, but it does seem understood that you can only have one <enclosure> item. Nonetheless, we found some podcast providers that go ahead and put multiple <enclosure> tags in their feeds, and we had to add new code to prefer one that matches /^audio\/*/