Over here on friendica 2024.12 the audio isn't displayed either.

I had expected that this would be some kind of activityPub convention of transmitting information about if a video or audio format is part of the content.

If I'm not wrong, when posting a mp3 URL like the following, the audio player is displayed right away:
https://media.tupambae.com/audio/bauhelmPodcast_welcomeFuture_orig.mp3
(well, in the preview that's not the case)

@utopiArte @Castopod :podcasting2:
The post from castopod is encoded as a "Note" ActivityStream object:

{
    "@context": "https://www.w3.org/ns/activitystreams",
    "attributedTo": "https://cast.tupambae.com/@bitPickup",
    "cc": [
        "https://cast.tupambae.com/@bitPickup/followers"
    ],
    "content": "<a href=\"https://cast.tupambae.com/@bitPickup/episodes/cc0-nc\">CC0-NC</a><br/>Moin, moin.\r<br />\r<br />Hier ein erster Vorgeschmack.",
    "id": "https://cast.tupambae.com/@bitPickup/posts/d88b27ab-c8c2-461d-9ec6-ac684af2908f",
    "published": "2025-01-22T04:00:01+00:00",
    "replies": "https://cast.tupambae.com/@bitPickup/posts/d88b27ab-c8c2-461d-9ec6-ac684af2908f/replies",
    "to": [
        "https://www.w3.org/ns/activitystreams#Public"
    ],
    "type": "Note"
}

there is no indication of something that should be played, there is only the link in the text content.
Now, mastodon get the first link it find in post and try tho show a preview, I think using OpenGraph.
Friendica sometimes does it also, I don't know the details and I don't know why it doesn't show a preview for this. I'm pretty sure that it will not show a player anyway.

The point is: there is nothing in that Note object which says "it's a podcast". It's just a note that an episode is published, with a link to it. The fact that mastodon show the player is unrelated from the fact that the note links to a podcast. I'm sure the fact the link is the first thing in the note is a deliberated choice by castopod devs, to take advantage of Mastodon's behavior

I had expected that this would be some kind of activityPub convention of transmitting information about if a video or audio format is part of the content.

Yes there is: attachment property. But then Friendica will shows attachments as icons at the end of the post. Still without a player.

Castopod devs chose to create their ActivityStream object of type 'PodcastEpisode', which you can get requesting the AS version of the episode page

$ curl -s -H Accept:application/activity+json https://cast.tupambae.com/@bitPickup/episodes/cc0-nc | jq .
{
  "@context": "https://www.w3.org/ns/activitystreams",
  "id": "https://cast.tupambae.com/@bitPickup/episodes/cc0-nc",
  "type": "PodcastEpisode",
  "published": "2025-01-22T04:00:01+00:00",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "cc": [
    "https://cast.tupambae.com/@bitPickup/followers"
  ],
  "attributedTo": "https://cast.tupambae.com/@bitPickup",
  "comments": "https://cast.tupambae.com/@bitPickup/episodes/cc0-nc/comments",
  "description": {
    "type": "Note",
    "mediaType": "text/markdown",
    "content": "The basics",
    "contentMap": {
      "en": "<p>The basics</p>\n<footer><p><a href=\"https://troet.cafe/@bitpickup\">bitPickup contact</a> in the fediVerse</p>\n</footer>"
    }
  },
  "image": {
    "type": "Image",
    "mediaType": "image/png",
    "url": "https://cast.tupambae.com/media/podcasts/bitPickup/cover_feed.png"
  },
  "audio": {
    "id": "https://cast.tupambae.com/audio/@bitPickup/cc0-nc.m4a",
    "type": "Audio",
    "name": "CC0-NC",
    "size": 161445,
    "duration": 7.808,
    "url": {
      "href": "https://cast.tupambae.com/audio/@bitPickup/cc0-nc.m4a",
      "type": "Link",
      "mediaType": "audio/mp4"
    }
  }
}

But this is not a standard object, and Friendica (and I think many other services) does not know how to display it, failing back to render the "content", losing the link to the audio file.

A note with a link is a reasonable way to post something understandable by everyone.

@Castopod :podcasting2: @utopiArte Mh, quick correction. if the link to the audio file is in an "attachment" property, friendica will show the player. Just as in this post

But from a semantic prospective (acting like I know what it means), the audio file is not attached to the note, but to the episode.