Embedding Youtube video on comments

Just add the comments to oEmbed. Here’s a small plugin that you can use as MU-Plugin or normal plugin and that should explain what’s going on pretty well.

<?php
defined( 'ABSPATH' ) or exit;
/* Plugin Name: (#105942) oEmbed Comments */

add_filter( 'comment_text', 'wpse_105942_oembed_comments', 0 );
function wpse_105942_oembed_comments( $comment )
{
    add_filter( 'embed_oembed_discover', '__return_false', 999 );

    $comment = $GLOBALS['wp_embed']->autoembed( $comment );

    remove_filter( 'embed_oembed_discover', '__return_false', 999 );

    return $comment;
}