The spectrogram plugin for WaveSurfer.js now supports frequency labels. I've implemented this to the master library and will be available on version 1.3.2.  

How to enable to frequency labels:

Step 1 - create the WaveSurfer object

var wavesurfer = WaveSurfer.create({
  container: '#waveform',
  waveColor: '#D2EDD4',
  progressColor: '#46B54D'
});

Step 2 - create the Spectrogram plugin inside the wavesurfer 'ready' event

wavesurfer.on('ready', function () {
  var spectrogram = Object.create(WaveSurfer.Spectrogram);
  spectrogram.init({
    wavesurfer: wavesurfer,
    container: "#wave-spectrogram",
    fftSamples: 1024,
    labels: true
  });
});

Step 3 - load some music

wavesurfer.load('http://wavesurfer-js.org/example/media/demo.wav');