This question gets asked an awful lot, so here's a quick how-to on embedding video in a post.
First, you need to copy HTML coding to embed it. YouTube lists an embed code at the bottom of the video description, to the right of the viewer. Google has a button near the top, that says "Email/Embed" then "HTML Embed" under a tab. Both are easy enough to find, and when you've found them, copy the entire thing.
On LttD, you must go into full reply mode rather than quick reply mode (full reply mode is automatically used when making a new thread), then in the drop-down under the post field for "Markup," switch to "Using HTML." I recommend against using both UBBCode and HTML since this has a tendency to cause misinterpretations of some codes and may screw up your video embed.
Once HTML is enabled, just paste the embed code you got from the video host, and add text as you see fit. Be aware that you may not use the UBBCode markup when doing so (unless you enabled both which I recommend against), so things like [i]italics[/i] and [b]bold[/b] will not work - this post itself is HTML enabled but not UBBCode, and as you can see the markup just fails. You MUST use [br] replacing [ with < and ] with > when making line breaks, ENTER no longer works!
Please do NOT embed autoplay video or music, that is extremely irritating for users using flat mode or for people who may have speakers turned up and don't want to be assaulted with your media before they're prepared for it.
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."