The easiest method of adding background sound is to include a line like this in your blog template, just after the opening <body> tag:
<bgsound src="http://yoursite.com/yoursound.mp3" loop=infinite>
where the URL is that of the background music you wish to use. This method works perfectly well, but visitors to your site will not be able to switch the music off if it becomes distracting.
So instead, you may prefer to use this method instead, which is compatible with all browsers (press the play icon to start music):This is achieved by including the following code somewhere in your template (eg: the sidebar):
<embed width="200" src="http://yoursite.com/yourmusic.mp3" autostart="false" loop="false" height="50"></embed>
You can change some of the variables to suit your needs:
autostart: set to "true" to make the music begin as soon as the page is loaded, or "false" to ensure the user has to press the play icon
loop: set to "true" to have the music play over and over, or "false" so that it plays only once.
height: if this is set to "40", only the play options will be visible. You can make this larger if you prefer, in which case blank space will be visible.
Using either method, you can link to .mp3, .mid or .wav music files. However, you must be aware of copyright restrictions, and ensure you have the appropriate permission for use for the music you decide to use!
Here are some sites I've used for copyright-free music you could use in your blog:
MonkeyFilter Wiki: contains links to many sites offering free music downloads.