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I'm having some trouble with this. I've taken the snippet you gave me, plus some of the documentation on rwmb_set_values, put this inside init with priority 99 as recommended. This will be a one-time process and then I will remove it. But the rmwb_set_values function is doing nothing.

add_action( 'init', function() {

    $sermons_a = array(
        'post_type' => 'message',
        'order' => 'DESC',
        'nopaging' => true,
    );

    $sermons_query = new WP_Query( $sermons_a );

    if ( $sermons_query->have_posts() ) {
        while ( $sermons_query->have_posts() ) {
            $sermons_query->the_post();

            // Get existing data
            $temp_audio = rwmb_meta( 'audio', '', get_the_ID() );
            $temp_video = rwmb_meta( 'video-id', '', get_the_ID() );
            $speakers = get_the_terms( get_the_ID(), 'speaker' );
         
            $groups = array(); // since this is a cloneable field, create an array of group values
            $group = array(); // individual group value
            $group['message_media_campus'] = 12; // literal value here
            if ($speakers) {
                $group['message_media_speaker'] = $speakers[0]->term_id; // only using the first speaker
            }

            $group['message_media_audio'] = $temp_audio;
            $group['message_media_video-id'] = $temp_video;

            $groups[] = $group;

            $group_serialize = maybe_serialize( $groups );

            rwmb_set_meta( get_the_ID(), 'message_media_group', $group_serialize );

        } // end while
    } // endif
}, 99 );