1.Log in to your dashboard--> layout- -> Edit HTML
2.Scroll down to where you see <head> tag.
3.Copy below code and paste it just after <head> tag:
<bgsound src="YOUR-AUDIO-FILE-LINK" loop="-1"/>
Replace "YOUR-AUDIO-FILE-LINK" with your real audio file link.
4.Now save your template and you are done.
Note: This tip works only in internet explorer.