Assuming you are loaing external SWFs for the tracks, one way to do this is to compare the url of the file already playing to the one you intend to play.

Code:
// button for track1
on(release){
   // if the url you want to load is not a substring of the url
   // currently loaded in _level1, then go ahead and load new song.
   // Otherwise the song in _level1 is song1, do not reload and restart
   if(_level1._url.indexOf("urltosong1.swf") == -1){
        _level1.unloadMovie(); // clear old song
        loadMovieNum("urltosong1.swf",1); // load new song
   }
}