I don't know if there's already a tutorial of this somewhere but some of you may know this Youtube Player tutorial already:
I think I discovered the simplified version of this which works for non-premium users too!
Try pasting this in your Custom Box, Newest Deviations or Deviantart ID:
<da:embed id="VIDEO_ID" profile="youtube" width="500" height="25">
Or copy and paste this working example on your profile:
<da:embed id="UvfKkBkcBXs" profile="youtube" width="500" height="25">
Place the Youtube video ID in the "VIDEO_ID" to add your own music to your custom box.
You can also change the length of the player by changing the width="500" to width="250" or width="50" for example.
EDIT 27 NOVEMBRE 2015:
Hi Again! Some of you may have noticed the Youtube player isn't showing the same way as it used to be.
That is because Youtube (one again) updated it's Youtube player.
With the height from the code ab