There are a few podcast platforms, like NPR One and Spotify, that require you to submit a secure feed (HTTPS) rather than a non-secured feed (HTTP). You should always try to have a secure feed on hand.
All the feeds that PRX generates are secure. But since you are running this feed through Feedburner you will need to make sure your feedproxy is secure as well.
You can setup a secure and non-secured feed using Feedburner’s My Brand tool. You can find more information on setting up MyBrand here (you must be logged in to access the page). Your MyBrand feed will be you non-secure feed, your Feedburner feed will be your secure feed (see examples below).