External Sound Method:
var sndVarName:Sound = new Sound();
sndVarName.load(new URLRequest("path_to_file.mp3")
sndVarName.play();
Pro: Quicker SWF load time
Con: Delay in initially playing
This method has a delay when before the sound plays, but reduces load-time of the flash swf since the file is the embedded.
Internal Sound Method
1. Right click on the library Item
2. Select properties
3. Check "export for actionscript"
4. Give a class name, usually something like Myclassname, uppercase with the first letter. I usually use names that aren't likely to be reserved.
var sndVarName:Clicker = new Clicker();
sndVarName.play();