And that's it. You've now added a basic looping audio background to your animation. You can leave it as-is, or you can tweak it using the audio settings on the Properties bar; you can set various effects, such as "Fade Left to Right" or vice versa.
If you leave the Synch on "Event" and the "Loop" on 0, it will play continuously--but it will also restart another instance each time your animation loops, so you'll end up with a sort of "round" effect as the song plays in staggered layers. Setting it to "Start" triggers the sound when the animation starts, but it will continue after the animation is over or through the animation's loops, then stop until the end of the current animation loop only to restart when the animation starts again. I've set mine to "Stream" with a loop count of "0", so that the sound plays for as long as the animation does and then starts over when the animation does--a little jarring at the moment, but as we extend the animation in further lessons it will work better.
There are actionscript controls to better control your audio, but those are for more advanced lessons. For now this is enough to set your background music.