Any ideas?

Doesn't Work.js

Listener.prototype.getPeakFrequency = function() {
  // Find where to start.
  var start = this.freqToIndex(this.options.codec.options.minFreq);

  // TODO: use first derivative to find the peaks, and then find the largest peak.
  // Just do a max over the set.  However this doesn't seem to be reliable
   var max = _.max(this.frequencies);
   if (max > this.options.peakThreshold) {
     return this.indexToFreq(_.indexOf(this.frequencies, max));
  }
  return null;
};

Works.js

Listener.prototype.getPeakFrequency = function() {
  
  var start = this.freqToIndex(this.options.codec.options.minFreq);
  
  var max = -Infinity;
  var index = -1;

  for (var i = start; i < this.frequencies.length; i++) {
    if (this.frequencies[i] > max) {
      max = this.frequencies[i];
      index = i;
    }
  }
  // Only care about sufficiently tall peaks.
  if (max > this.options.peakThreshold) {
    return this.indexToFreq(index);
  }
  return null;
};

My JS is a bit rusty, but would do something like

Listener.prototype.getPeakFrequency = function() {
  var start    = this.freqToIndex(this.options.codec.options.minFreq);
  var maxindex = _.reduce(this.frequencies, function(acc,freq,index) {
    return (index >= start && freq > maxindex[0]) ? [freq,index] : acc;
  }, [-Infinity, -1]);

  // Only care about sufficiently tall peaks.
  return maxindex[0] > this.options.peakThreshold ? this.indexToFreq(maxindex[1]) : null;
};

Ooops, try again (bug in the if() inside _.reduce

Listener.prototype.getPeakFrequency = function() {
  var start    = this.freqToIndex(this.options.codec.options.minFreq);
  var maxindex = _.reduce(this.frequencies, function(acc,freq,index) {
    return (index >= start && freq > acc[0]) ? [freq,index] : acc;
  }, [-Infinity, -1]);

  // Only care about sufficiently tall peaks.
  return maxindex[0] > this.options.peakThreshold ? this.indexToFreq(maxindex[1]) : null;
};