The specific multimedia code that you use will be given to you when I give you your profile, or it is located below the coding on the freebie post, if the profile is a freebie that you are using.
First, find your desired youtube video with the music you want. Secondly, copy the video's coding. The coding is the funky letters after the '=' in the url box. Right here:
After copying it, insert/replace it with the 'OOOOO' in the coding given to you.
Lastly, and optionally, you can take out the '&autoplay=1' to stop it from automatically playing when you view the profile.
Pop it in your multimedia box on your profile options page and you're good to go!