Well, looking at what your URL links to, and right now I'm using an Android tablet, I see this:

https://www.worldmag.com/podcast/worldandeverything.xml?type=m4a

You didn't use Thunderbird to post that link, and I didn't use Thunderbird to view it, so I don't think it's reasonable to accuse Thunderbird of changing it.

Looking at the source at

http://www.worldmag.com/podcast/worldandeverything.xml

I see these lines at the top

<rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" xmlns:atom="http://www.w3.org/2005/Atom" version="2.0">

 <channel>
   <title>The World and Everything in It</title>
   <itunes:subtitle>The World and Everything in It</itunes:subtitle>
   <link>http://www.worldmag.com/</link>
   <atom:link href="https://www.worldmag.com/podcast/worldandeverything.xml" rel="self" type="application/rss+xml"/>
   <language>en-us</language>
   <copyright>2015 God's World Publications</copyright>
   <itunes:author>WORLD Radio</itunes:author>
   <itunes:summary>'The World and Everything in It' is a Monday-Friday audio news magazine produced by WORLD Radio. The program features reports, interviews, and analysis from WORLD News Group's editorial team. WORLD is your source for news and views presented from a biblical worldview perspective.</itunes:summary>
   <pubDate>Fri, 28 Aug 2015 04:00:00 -0400</pubDate>
   <lastBuildDate>Fri, 28 Aug 2015 04:00:00 -0400</lastBuildDate>
   <image>
     <url>http://www.worldmag.com//media/images/podcast-logos/twe_stack.png</url>
     <title>The World and Everything in It</title>
     <link>http://www.worldmag.com/</link>
   </image>
   <ttl>60</ttl>
   <description>'The World and Everything in It' is a Monday-Friday audio news magazine produced by WORLD Radio. The program features reports, interviews, and analysis from WORLD News Group's editorial team. WORLD is your source for news and views presented from a biblical worldview perspective.</description>
   <itunes:keywords>politics, news, movies, sports, radio, Christian, worldview, </itunes:keywords>
   <itunes:owner>
     <itunes:name>WORLD Radio</itunes:name>
     <itunes:email>webeditor@worldmag.com</itunes:email>
   </itunes:owner>
   <itunes:image href="http://www.worldmag.com/media/images/podcast-logos/twe_stack.png"/>
   <itunes:new-feed-url>https://www.worldmag.com/podcast/worldandeverything.xml?type=m4a</itunes:new-feed-url>
   <itunes:category text="News & Politics"/>
   <itunes:explicit>no</itunes:explicit>

There are some lines that make me suspicious that Thunderbird is changing the url. This is the line where I think the url change is coming from.

<itunes:new-feed-url>https://www.worldmag.com/podcast/worldandeverything.xml?type=m4a</itunes:new-feed-url>

Here is the line that I think Thunderbird should be using.

<atom:link href="https://www.worldmag.com/podcast/worldandeverything.xml" rel="self" type="application/rss+xml"/>

I don't know how feed readers are supposed to interpret a feed page, so I don't know why Thunderbird would be using the itunes:new-feed-url (Is that spelled correctly? Maybe that is the problem?)

Thanks

I added the linked FEED to Thunderbird daily. I got 200 itens and when I click on them I get sound files with an MP3 extension. When I click play it plays.

Thanks for the idea. I tried it and seemed to work at first. But the next time I opened Thunderbird the url had been changed to include type=m4a and another 200 posts had been downloaded with the m4a version of the podcast attached.

I find this:

• Add the feed and the first time you get mp3 format attachments.
• Delete all the messages and refresh and this time they are mp4 format.

I think there is a tracking mechanism so that when you delete a message, it won't be downloaded again. Clearly, you don't delete it on the server, as you do with email, since it would be removed for all other users. So something records the fact that you have downloaded it.

In this case, the fact that you have deleted it perhaps leads it to offer alternative formats.

ok This morning I have a second set of entries in the list in the form of M4A.

So I tried subscribing with Chromium. and one of it's feed readers. I was told that podcasts are onlty available to members. So I think what we are seeing is some sort of defective refusal to deliver to a non member.

Yes, Thunderbird will update an existing feed url if it finds an <itunes:new-feed-url> tag in the file; this feature was implemented in Tb 38. See the guide: https://support.mozilla.org/en-US/kb/how-subscribe-news-feeds-and-blogs

Inform the publisher that if the new url isn't valid or an update to it isn't desired, then the tag shouldn't be there. This is completely up to them

Thanks Alta I learned something today.

I contacted the publisher and the response was that this is a non-standard behavior in Thunderbird. Frankly, I agree. There ought to at least be an option to disable the tag. I plan to suggest as such in the bug report, https://bugzilla.mozilla.org/show_bug.cgi?id=304917

That bug is closed, if you want to modify the existing behavior file a new bug referencing the bug that created the action you want modified.

Thanks, done.

https://bugzilla.mozilla.org/show_bug.cgi?id=1200874

The bug I submitted was rejected. However, I fixed it myself with a little hacking. If anyone else has this problem I can tell you what you need to do.