I am able to paste a Youtube link into a post and have it auto embed just fine, but if I go to edit a media description, and put the link there, and then view the attachment page for that media, it just makes the URL clickable.
I have tried using just the link, the embed shortcode, and Jetpack’s youtube shortcode. All of them work on posts and pages, but not attachment content. I have tried using apply_filters('the_content', $img->post_content); instead of just the_content, but this doesn’t work either.