code: function(track, beatInd) {
    var event = clock.callbackAtTime(function(event) {
      var bufferNode = soundBank[track].createNode()
      bufferNode.start(event.deadline)
    }, nextBeatTime(beatInd))
    event.repeat(barDur)
    event.tolerance({late: 0.01})
    beats[track][beatInd] = event
  }
docs: This function activates the beat `beatInd` of `track`.