bbp_update_topic_voice_count( int $topic_id )

function bbp_update_topic_voice_count( $topic_id = 0 ) {
	global $wpdb;

	// If it's a reply, then get the parent (topic id)
	if ( bbp_is_reply( $topic_id ) ) {
		$topic_id = bbp_get_reply_topic_id( $topic_id );
	} elseif ( bbp_is_topic( $topic_id ) ) {
		$topic_id = bbp_get_topic_id( $topic_id );
	} else {
		return;
	}

	// Query the DB to get members in this topic
	$voices = $wpdb->get_col( $wpdb->prepare( "SELECT COUNT( DISTINCT post_author ) FROM {$wpdb->posts} WHERE ( post_parent = %d AND post_status = '%s' AND post_type = '%s' ) OR ( ID = %d AND post_type = '%s' );", $topic_id, bbp_get_public_status_id(), bbp_get_reply_post_type(), $topic_id, bbp_get_topic_post_type() ) );

	// If there's an error, make sure we have at least have 1 voice
	$voices = ( empty( $voices ) || is_wp_error( $voices ) ) ? 1 : $voices[0];

	// Update the voice count for this topic id
	update_post_meta( $topic_id, '_bbp_voice_count', (int) $voices );

	return apply_filters( 'bbp_update_topic_voice_count', (int) $voices, $topic_id );
}