You should be able to gracefully handle feeds in which only some of the items have enclosures.
Like the feed for this blog.
Suggested implementation: ignore them if your client really is only for listening to podcasts.
RSS is a medium that can transmit lots of different kinds of data. If you only care about one kind, just ignore the others. That's the philosophy of XML, as I understood it, when I developed the technology behind podcasting.
Keep on truckin!
PS: This post has a podcast attached, as a demo.
PPS: Another possibility, make it a preference setting for the user. Default to ignoring posts without enclosures, but make it easy for the user to override your choice.