Like you, I'm a newbee. However, I have managed to add background music in the form of an mp3 called "LuxsBoogie".
Here is what I did:

Create a "place holder' (a simple shape). Select the shape (placeholder), then:

> Go to "Utilities"
> Go to "Web Properties" in the menu (the shortcut is "SHFT + CTRL +W")
> Go to the "Placeholder" tab
> Go to HTML header button and insert the following:

<EMBED src="index_htm_files/LuxsBoogie.mp3"
autostart=true loop=false
volume=100 hidden=true>

Obviously your mp3 or WAV file will be called something else (not LuxsBoogie".
The placeholder will become invisible when viewed on the web.
However you can add a graphic on top of the place holder.
I added an animated gif.