After seeing some with music on their profiles, I remember asking Grey Wolf 777 how to do it, and this was the reply I got.

Well, there's a website called http://www.mp3raid.com. You can search for almost any song and there is a good guarentee that they will have it. Just search for the song, after you click on it you'll have to put in some code to make sure you can read (I really don't know why) and you click on the song of your choice and the next page pops up. There's a player on the page playing the song. Now you can post the song on your profile by copying the embed code under the player and pasting it on your profile, and there you go. You can also do it with image, art and fan fic discriptions.

I used the site mentioned here for putting a song in my profile.

As Mighty said, I do believe it's allowed, but it slows down people's computers, therefore, not the best thing for some members.

And of course, it can't be offensive music or have any other content that goes against the rules.

Yes, but It's not encouraged. Some people have slow computers and causes them to be slow or crash.

Yes, your allowed to! You have to get an HTML coded music player though!