well there are two ways you can do what you want:
1. Easier but less control : Make a movieClip and drop the soundfile in the first frame of the movieClip (or use the properties panel for the first frame). Add a stop action (stop() to the first frame. Then make a keyframe (in the second frame). Add a stopAllSounds (stopAllSounds() action to this second frame. After that add a stop action to the second frame. So now you have one frame with a sound file playing and a second frame with a stopAllSounds action (which of course stops the sound from playing... but also stops all other playing sounds). Now drag and drop the movieClip onto the main Scene. Name the movieClip something (like "music" or something). You do this using the properties panel. The sound should start playing as soon as the movie hits the frame in which you put the movieClip (judging by what you said, this should be frame 1).
You can stop the music by just making a button (outside of the movieClip) and adding the following code to it :
To play the music again you can duplicate the button and change the code to read:
Well there's the easy way. Now you cant control volume, or panning or pausing etc. The "harder" method lets you do this. Please reply if you need volume and stuff too.. Cause I dont wanna explain the whole thing here if not needed.
Hope I helped.