a moombah fork :)

beat2.js

var bpm = 124;
var tuning = 440;
var transpose = 12;

// constants
var tau = 2 * Math.PI; 

// adjust tuning to bpm
tuning *= 120 / bpm;

var rhythm = [
  [kicks,  "x...x...x...x..."],
  [snares, "...x..x....x..x....x..x....x..xx"],
  [hats,   "..x....x..x....x"],
  [bleep,  "127.8."]
];

function kicks(t, b) {
  return 0.8 * clip(env2(1/16,  9*sin(30, t), 35, 44, b), 0.5);
}

function snares(t, b) {
  return clip(env2(1/16,  9*sin(70, t) + Noise() *1.5 , 35, 44, b), 0.3);
}

function hats(t, b) {
  return clip(env2(1/16, tri(1098 + Noise(), b), 120, 20, b), 0.2);
}

function bleep(t, b, k) {
  return .35 * (Math.sin(tau*t*110/2*k) > 0.6) * Math.exp(-b*20) ;
}

var beat = new BeatBox(rhythm, 1/16);

function dsp(t) {
  var b;
  b = t * bpm / 120;
  return beat.run(t, b) * 0.5;
}

function BeatBox(patterns, div) {
  this.div = div;
  this.blocks = [];
  var k = patterns.length;
  for (var j = 0; j < k; j++)
  {
    var pattern = patterns[j][1];
    var seq = [];
    var l = pattern.length;
    for (var i = 0; i < l; i++)
    {
      var code = pattern[i];
      var val = null;
      if (code==='x')  { val = 1; } else if(code==='-') { val = 0.1; }
      if (/[0-9]/.test(code)) val = parseInt(code);
      
      seq.push(val);
    }
    this.blocks.push({seq:seq, start:0, rst:0, fn:patterns[j][0]});
  }
}

BeatBox.prototype.run = function(t, b) {
  var sum = 0;
  var measure = this.div;
  var k = this.blocks.length;

  for (var j = 0; j < k; j++) {
    var block = this.blocks[j];
    var idx = Math.floor(b / measure / 2 | 0);
    var sig = sequence(measure, block.seq, b);
    if(block.rst != idx && sig !== null) block.start = t;
    block.rst = idx;
    var start = t - block.start;
    sum += block.fn(t, start, sig);
  }
  return sum;
};

function sequence(measure, seq, t){
  return seq[(t / measure / 2 | 0) % seq.length];
}

function env2(measure, x, y, z, t){
  if(t > measure) return 0;
  return env(measure, x, y, z, t);
}

function env(measure, x, y, z, t){
  var ts = t / 2 % measure;
  return Math.sin(x * (Math.exp(-ts * y))) * Math.exp(-ts * z);
}

function clip(x,max) {
  return Math.min(max, Math.max(x, -max));
}

function sin(x, t){
  return Math.sin(tau * t * x);
}

function saw(x, t){
  return 1-2 * (t % (1/x)) * x;
}

function tri(x, t) {
  return Math.abs(1 - (2 * t * x) % 2) * 2 - 1;
}

function Noise() {
  return Math.random() * 2 - 1;
}

function sqr(x, t) {
  return sin(x, t) > 0 ? 1 : -1;
}