Unit testing with Mocha+PhantomJS async code

I'm new to unit testing, so please forrgive me if my question could be silly. I wrote an unit test using Mocha with PhantomJS and Chai as assertion library. The code that I want to test is the following function:

function speakingNotification(audioStream){
  var options = {};
  var speechEvents = hark(audioStream, options);

  speechEvents.on('speaking', function() {
    return 'speaking';
  });

  speechEvents.on('stopped_speaking', function() {
    return 'stopped_speaking';
  });
}

As you can see it takes an audioStream parameter as input and then use a librabry called hark.js http://ift.tt/1zowatl for detecting speaking events. The function should return if the user is speaking or not.

So I wrote the following unit test:

describe('Testing speaking notification', function () {
    describe('Sender', function(){

        var audio = document.createElement('audio');
        audio.src = 'data:audio/mp3;base64,//OkVA...'; //audio file with sound

        var noAudio = document.createElement('audio');
        noAudio.src = 'data:audio/mp3;base64,...';  //audio file with no sound

        it('should have a function named "speakingNotification"', function() {
            expect(speakingNotification).to.be.a('function');
        });

        it('speaking event', function () {
            var a = speakingNotification(audio);
            this.timeout( 10000 );
            expect(a).to.equal('speaking');
        });

        it('stoppedSpeaking event', function () {
            var a = speakingNotification(noAudio);
            this.timeout( 10000 );
            expect(a).to.equal('stopped_speaking');
        });

    });
});

The test fails and shows:

 AssertionError: expected undefined to equal 'speaking'

 AssertionError: expected undefined to equal 'stopped_speaking'

I also tried to use done() insted of the timeout, however the test fails and shows:

ReferenceError: Can't find variable: done

I searched for tutorials, however I can only find simple examples that don't help. How can I write a correct test?