mp3s (or any music for that matter) + playing in the backgrond of a website automaticly without giving the browser much of a way to control it = one of the top 1 bad things that people do to their website, especially with mp3s. reasons being (1) people most of the time have their personal media player already going on their computer and (2) there are still 56k and even 28.8k modem users out there. For them, downloading mp3s is quite an adventure, and one embedded in a page would make it take forever to load.
The best solution is this: upload your song to
http://www.savefile.com and post a link to the song in your xanga somewhere noticible on the page and tell people it's really kewl. This will keep everyone happy.
But truly, <embed src="*.mp3" etc> stuff should only be used for flash images and stuff like that. If you really need to have that song up there tho, do it midi style. I'll even host the midi on my site for you and you can direct-link to it from your xanga.