You can add background music to your journal via the use of a custom S1 or S2 style. Creating and using custom styles is a paid account benefit. See http://www.livejournal.com/support/faqbrowse.bml?faqid=21 for details on purchasing a paid account.

A tutorial on background music in Style System 1 is given in the HowTo journal at http://www.livejournal.com/users/howto/3570.html. Using this tutorial in S1 requires that you create a custom style, as outlined in http://www.livejournal.com/support/faqbrowse.bml?faqid=128.

To use background music in Style System 2, you must create a custom S2 style and adapt the code given for Style System 1. See http://www.livejournal.com/support/faqbrowse.bml?faqid=177 for information on custom S2 styles.

These are the only supported methods of adding music to your journal. It may be possible to produce this effect in other ways, but these other ways often lead to problems, as they use non-standard and unsupported HTML. If you have attempted to add music to your journal through another method, your journal may display incorrectly to you or other viewers of your journal. LiveJournal Support is unable to offer assistance on unsupported methods not listed here.

Note that the background music you use must be hosted by a webspace provider that allows remote loading to LiveJournal. Full details on this issue can be found at http://www.livejournal.com/support/faqbrowse.bml?faqid=130.

If you choose to add background music to your journal, you must ensure that you have permission to use the music in this way, either by gaining permission from the copyright holder or by ensuring that the music is not covered by copyright. LiveJournal cannot give you permission to use someone else's work.


FURTHER READING

The HowTo journal:
http://www.livejournal.com/userinfo.bml?user=howto

Advanced S2 customization resources:
http://www.livejournal.com/support/faqbrowse.bml?faqid=172