Javascript load and play audio files

tracks.js

function TrackManager(trackFiles, onloadCallback){

    var audioContext,
        tracks = {},
        loadedTracks = [];

    window.AudioContext = window.AudioContext||window.webkitAudioContext;
    audio_context = new AudioContext();

    function load_track(name, url){
        var request = new XMLHttpRequest();
        request.open('GET', url, true);
        request.responseType = 'arraybuffer';
        request.onload = function(e) {
            initSound(name, e.target.response);
            console.log('loaded track ' + name);

            loadedTracks.push(trackName);
            if (loadedTracks.length === trackFiles.length) {
                return onloadCallback();
            }
        };
        request.send();
    }

    function initSound(trackName, arrayBuffer) {
        audio_context.decodeAudioData(arrayBuffer, function(buffer) {
            tracks[trackName] = buffer;
        }, function(e) {
            console.log('Error decoding', e);
        }); 
    }

    for (trackName in trackFiles) {
        loadTrack(trackName, trackFiles[trackName]);
    }

    this.playTrack = function(trackName){
        var source;

        if (loadedTracks.indexOf(trackName) === -1) {
            return false;
        }

        source = audio_context.createBufferSource();
        source.buffer = tracks[trackName];
        source.loop = false;
        source.connect(audio_context.destination);
        source.noteOn(0);
        return source;
    }
}


var sound,
    tracks = {
        track1: 'media/audio/track1.m4a',
        track2: 'media/audio/track2.m4a',
        track3: 'media/audio/track3.m4a'
    };

function allTracksLoaded(){
    console.log('all tracks loaded');
    track = sound.playTrack('track1');
    track.stop();
}

sound = new TrackManager(tracks, allTracksLoaded);