Subset of the code included in DanQ.me's theme, providing podcasting functionality into an RSS feed. See https://danq.me/dan-q-the-podcast for further explanation.

q23-podcasting-extensions.php

<?php
define('PODCAST_TAG', 'dancast');
define('PODCAST_DEFAULT_IMAGE', 'https://danq.me/wp-content/uploads/2024/05/podcast-art.jpg');
define('PODCAST_SUMMARY', "A podcast nobody asked for about things only Dan cares about. A spin-off of the blog of Dan Q, who's been making random stuff on the Internet since the 1990s.");
define('PODCAST_URL', 'https://danq.me/tag/dancast/');
define('PODCAST_TITLE', 'Dan Q - The Podcast');

/**
 * If the 'podcast_link' postmeta is filled, advertise it appropriately in RSS:
 */
function q23_podcast_in_rss() {
  // Don't apply when not requesting a particular tag, in a feed
  if ( ! is_feed() || ! is_tag(PODCAST_TAG) ) return;
  // Get the podcast link
  global $wp_query;
  $postid = $wp_query->post->ID;
  $podcast_link = get_post_meta($postid, 'podcast_link', true);
  $podcast_duration = get_post_meta($postid, 'podcast_duration', true);
  $podcast_image = get_post_meta($postid, 'podcast_image', true);
  $podcast_explicit = get_post_meta($postid, 'podcast_explicit', true);
  // If it's blank, return the content as-is:
  if ( empty($podcast_link) ) return;
  // Ensure the file exists!
  $filepath = $_SERVER['DOCUMENT_ROOT'] . '/wp-content/uploads/'. $podcast_link;
  if ( ! file_exists( $filepath ) ) return;
  // Get its size in bytes:
  $filesize = filesize($filepath);
  // Otherwise, add it to the content:
  ?>
    <enclosure url="https://danq.me/_q23u/<?php echo $podcast_link; ?>" length="<?php echo $filesize; ?>" type="audio/mpeg" />
    <?php if( ! empty( $podcast_duration ) ) echo "<itunes:duration>$podcast_duration</itunes:duration>"; ?>
    <?php if( ! empty( $podcast_image ) ) echo "<itunes:image href=\"$podcast_image\" />"; ?>
    <?php if( ! empty( $podcast_explicit ) ) echo "<itunes:explicit>$podcast_explicit</itunes:explicit>"; ?>
  <?php
}
add_action('rss2_item', 'q23_podcast_in_rss');

/**
 * Update the descriptions of podcasts in their RSS feed:
 */
function q23_podcast_in_rss_content( $output ) {
  // Don't apply when not requesting a particular tag, in a feed
  if ( ! is_feed() || ! is_tag(PODCAST_TAG) ) return $output;
  // Update the content:
  $permalink = get_the_permalink();
  $output = "Based on an original blog post at: $permalink<br><br>" . $output;
  $podcast_desc = get_post_meta( get_the_ID(), 'podcast_desc', true );
  if( ! empty( $podcast_desc ) ) $output = $output . "<br><br>" . nl2br( $podcast_desc );
  return $output;
}
add_action('the_excerpt_rss', 'q23_podcast_in_rss_content');

/**
 * Podcasts must only have one <enclosure>; let's ensure the others don't occur.
 */
function q23_podcast_remove_other_enclosures( $html_link_tag ) {
  // Don't apply when not requesting a particular tag, in a feed
  if ( ! is_feed() || ! is_tag(PODCAST_TAG) ) return $html_link_tag;
  return '';
}
add_filter('rss_enclosure', 'q23_podcast_remove_other_enclosures');

/**
 * Don't need full HTML content in a podcast feed.
 */
function q23_podcast_remove_content_encoded( $content ) {
  // Don't apply when not requesting a particular tag, in a feed
  if ( ! is_feed() || ! is_tag(PODCAST_TAG) ) return $content;
  return '';
}
add_filter('the_content_feed', 'q23_podcast_remove_content_encoded', 100);

/**
 * Add the iTunes namespace to the RSS feed:
 */
function q23_podcast_namespace() {
  // Don't apply when not requesting a particular tag, in a feed
  if ( ! is_feed() || ! is_tag(PODCAST_TAG) ) return;
  echo " xmlns:itunes=\"http://www.itunes.com/dtds/podcast-1.0.dtd\" xmlns:podcast=\"https://podcastindex.org/namespace/1.0\"\n";
}
add_action('rss2_ns', 'q23_podcast_namespace');

/**
 * Add the iTunes-specific tags to the RSS feed as a whole:
 */
function q23_podcast_head() {
  // Don't apply when not requesting a particular tag, in a feed
  if ( ! is_feed() || ! is_tag(PODCAST_TAG) ) return;
  ?>
    <podcast:locked>no</podcast:locked>
    <podcast:guid>6bd2323b-1060-504d-8d3c-5dbae5c420be</podcast:guid>
    <itunes:category text="Society &amp; Culture">
      <itunes:category text="Personal Journals"/>
    </itunes:category>
    <itunes:category text="Technology" />
    <itunes:image href="<?php echo PODCAST_DEFAULT_IMAGE; ?>" />
    <itunes:author>Dan Q</itunes:author>
    <itunes:type>episodic</itunes:type>
    <itunes:explicit>true</itunes:explicit>
    <copyright>© Dan Q <?php echo date('Y'); ?>; all original content CC-BY-AT; details at https://danq.me/license/</copyright>
    <podcast:license>https://danq.me/license/</podcast:license>
    <podcast:medium>blog</podcast:medium>
    <itunes:owner>
      <itunes:name>Dan Q</itunes:name>
      <itunes:email>podcast@danq.me</itunes:email>
    </itunes:owner>
  <?php
}
add_action('rss2_head', 'q23_podcast_head');

function q23_podcast_description( $current, $show ){
  if ( $show != 'description' ) return $current;
  if ( ! is_feed() || ! is_tag(PODCAST_TAG) ) return $current;
  return PODCAST_SUMMARY;
}
add_action('bloginfo_rss', 'q23_podcast_description', 10, 2);

function q23_get_podcast_url( $current, $show ){
  if ( $show != 'wpurl' && $show != 'url' && $show != 'siteurl' && $show != 'home' ) return $current;
  if ( ! is_feed() || ! is_tag(PODCAST_TAG) ) return $current;
  return PODCAST_URL;
}
add_action('get_bloginfo_rss', 'q23_get_podcast_url', 10, 2);

function q23_get_podcast_name( $current, $show ){
  if ( $show != 'name' ) return $current;
  if ( ! is_feed() || ! is_tag(PODCAST_TAG) ) return $current;
  return PODCAST_TITLE;
}
add_action('get_bloginfo_rss', 'q23_get_podcast_name', 10, 2);

function q23_podcast_title($title) {
  // Don't apply when not requesting a particular tag, in a feed
  if ( ! is_feed() || ! is_tag(PODCAST_TAG) ) return $title;
  return PODCAST_TITLE;
}
add_filter('wp_title_rss', 'q23_podcast_title');
add_action('get_wp_title_rss', 'q23_podcast_title');

This code above is released into the public domain under the Unlicense. Do whatever you like with it.