Code:
<embed src="melody.mid" width=2 height=0 autostart=true loop=true>
<noembed>
<bgsound src="melody.mid" loop=infinite>
</noembed>
That will work for, obviously, midi files and probably wav files (never tried myself). About the only other option for mp3's that I see is something like
tinyplayer, a mp3 applet. Notice the small green 2 button player towards the top. That's all that shows. You can change the bgcolor or it, but unless you want to rewrite the applet itself, I don't think you can change the appearance of it much.