function fluctuate(bar) {
  var hgt = Math.random() * 100 + 70;
  var t = hgt * 7.5;
  var marg = (170 - hgt) / 2;
  
  bar.animate({
      height: hgt,
      marginTop: marg
  }, t, function() {
      fluctuate($(this));
  });
}

$('#music-bars span').each(function(i) {
  fluctuate($(this));
});