You appear to be off to a good start. Here's how you deal with sound in Flash movies: first your audio file needs to be in the WAV format because Flash is finicky when it comes to MP3s, you'll want to import the sound to the library since it won't import into the timeline like photos. Then you'll drag and drop it to the timeline.
In the properties you'll notice a menu for settings like Stream, Start, Event & Stop. Be sure to pick Stream for linear movies, Start mode forces the audio to play to its full length before it'll play again, regardless of where you are in the timeline. Event will also force audio to play to its full length but it won't prevent new plays of the same audio, meaning that the audio can play in an overlap, multiple times if the frame length is short enough. Stop mode will literally stop the audio file no matter what setting you gave to it prior; the way that works is that you have a set of frames with the audio on either Stream, Start or Event mode and follow it up with a frame with the same audio that you want to stop under Stop mode.
We're also now going into audio optimization. When you look at you publish settings [under the File tab] you'll see two settings called Audio Stream and Audio Event. You can change either of these to enhance or compress the audio in your SWF when publishing. The settings I keep are MP3, 160KBPS, Stereo. If you'd like to save these settings for future publications, you're gonna have to create a new profile [the [+] symbol] and import the profile the next time you use it.
Next is the last thing I have to say about audio. Don't import files with more than 44,000 Hz, Flash can't properly handle anything above that threshold and will compensate by decelerating the audio.
Next is something of a universal convention, for the record, this is something to be expected on Newgrounds; get a preloader. You can get them for free right here: http://www.newgrounds.com/downloads/preloaders/
Add one of those to the first frame of your Flash movie and you're good to go. If you want the animation to continue looping and skip the preloader, add some ActionScript to the last frame of your movie: gotoAndPlay(2);
You've got better potential than most of the chucklefucks around here. Keep it up.