@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"
}

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"
    }
  }
}

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.

utopiArte

2025-01-23 18:50:49

Post of the audio MP3 URL link in BBCode audio brakets [audio][/audio]