Displaying Custom Fields & Custom Taxonomies in RSS Feeds?

I’m trying to display a custom field and a custom taxonomy in the RSS feeds.

I found the below code which dies what I want:

add_action('rss_item', 'rssCustomFieldContent');
add_action('rss2_item', 'rssCustomFieldContent');

function rssCustomFieldContent()
{
$thispostID = get_the_ID();
echo '<audio_link>' . get_post_meta($thispostID, 'wpcf-sermon_audio', true) . '</audio_link>';
$speaker = wp_get_post_terms($thispostID, 'teacher', array('fields' => 'names'));
echo '<speaker>' . $speaker[0] . '</speaker>';
}

I’m probably an idiot but I can’t figure out where to put it. I tried my theme’s functions.php but that didn’t seem to work…

Where do I put this code? Or is there an alternative way to get the same result?