We developed a fix to more aggressively add the enclosure media to the feed, but we haven't been able to confirm it fixes the problem. We believe the WishList member folks got wind of us working on a solution and thus revoked our license copy to WishList member before we were able to finish testing that our solution fixed the problem.
Just add the following near the top of your wp-config.php, then see if it fixes your feeds. We would greatly appreciate it if you could reply if it works or not.
define('POWERPRESS_ENCLOSURE_FIX', true);
To explain, PowerPress will then check to make sure the enclosure is in the feed. If it's not, PowerPress re-adds it. By default WordPress adds the media enclosure (not PowerPress) and from our testing WishList member plugin removes enclosures unless you're a member to the blog by design. So our solution goes against what WishList member is intended to do, but we believe this is what most podcasters intend to do, offer both a free and paid versions of their media to their listeners.