Code:
<EMBED src="file.mp3" autostart=true hidden=true>
But, using an MP3 like this could be annoying to your users as it may be a sizeable download. Also, it is important to note that the server may need to have the MIME properly configured.
If that doesn't work, try changing the file type from .mp3 to m3u and then using:
Code:
<EMBED src="file.m3u" autostart=true hidden=true>