I know it's ugly and annoying, and we really don't need a million journals playing a midi of the A-Team theme tune, but this is becoming a frequently asked question in the style systems support category. So we (erin, Tenshi and I) have come up with a proposed FAQ for background music. It covers S1 and S2, and addresses the paid status, hence it being in customization rather than s1/s2.
Adding background music to your journal requires the use of a custom style, regardless of the Style System (http://www.livejournal.com/support/faqbrowse.bml?faqid=173). Please see http://www.livejournal.com/support/faqbrowse.bml?faqid=21, "How do I buy a paid account?" for details on purchasing a paid account.
A tutorial on background music in Style System 1 exists in the HowTo journal (http://www.livejournal.com/userinfo.bml?user=howto) at http://www.livejournal.com/users/howto/3570.html. Using this tutorial in S1 requires a custom style as outlined here, http://www.livejournal.com/support/faqbrowse.bml?faqid=128.
Using background music in Style System 2 requires adaptation of this code in a custom style. Please see http://www.livejournal.com/support/faqbrowse.bml?faqid=177 for information on custom styles in Style System 2.
For further information on customising in Style System 2 please see http://www.livejournal.com/support/faqbrowse.bml?faqid=172, "Where can I go to learn more about advanced S2 customization?".
Remember, the background sound you use must be hosted on third party webspace that allows remote loading. Full details on this can be found at http://www.livejournal.com/support/faqbrowse.bml?faqid=130, "Why are my images not displaying? What is remote loading?".
Please make sure, if you choose to add background music to your journal, that the music is not covered by any copyrights or that you have permission from the copyright holder. The existence of this FAQ does not imply that LiveJournal can give you permission to use someone else's copyrighted work.
Please pick this apart and I'll edit the post with changes as necessary. For all that is sanity in the land of styles, we really kinda need this as an FAQ. Oh, and please can someone just read over for english/american english crossovers, since I tend to do that a bit. Thanks :)