This is a common issue.
You need to host your files on a hosting provider that supports Podcasting specifically - and that means ones that support byte-range requests.
Please check out - http://www.libsyn.com
Here is some info I found on this.
You will need to ask the folks at Wordpress.com for an official answer.
I just know we get many people signing up because of the Byte-Range request issue. Which we fully support.
Here is the message Blip.tv sent out to Audio podcasters.
At blip.tv we’re focused on being the best home for high quality, original, episodic, video web series. To that end, we have focused on making our upload process as easy and fool-proof as possible. For a number of years we’ve permitted the upload of non-video formats, but beginning December 13th, 2011 we will be turning off the ability to upload all audio-only formats, including MP3.
Pro accounts will still have the ability to create MP3 conversions from their source video files, but direct MP3 uploads will no longer be allowed. If you are uploading an audio only podcast, we strongly recommend using a more appropriate platform such as Libsyn or Podbean. By next week at the latest, we will begin offering an option in your Dashboard to re-direct your current feed to a new location. This will allow iTunes to move your subscribers over to your new feed and host automatically.
I had this problem with a podcast hosted at WPengine.com and using the PowerPress plugin.
I discovered that turning off the stats (Blubrry and PodTrac) allowed me to submit the podcast.
I'm guessing that the check Apple does to confirm support for Byte-Range support does not
understand the redirects used by the stats services.
From what I understand the problem lies with the server hosting the media files (not the feed), because it needs to be able to handle requests for part of the file. This would suggest DropBox; so unless you can persuade them to provide this facility you will need to find another hosting service that can.