Hangout Band basic structure (see http://www.allmyplus.com/hangout-band/)

hangout-band.js

(function (window) {
  "use strict";
  var audio, hangout_band, sounds;

  sounds = {
    // define various parameters for each note that can be played
  };
  
  function WebAudio() {
    // all web audio API stuff handled here
    
    this.AUDIO_SUPPORT = false;
    if (window.AudioContext || window.webkitAudioContext) {
      this.AUDIO_SUPPORT = true;
    }
    
    this.load_sound = function(nr, callback) {
      //...
    }
    
    this.play_sound = function(nr) {
      //...
    }
  }

  function HangoutBand() {
    this.sending = true;
    this.receiving = true;
    // define several more necessary variables here...

    // waiting for API to be ready...
    window.gapi.hangout.onApiReady.add(this.onApiReady.bind(this));
  }

  HangoutBand.prototype.onApiReady = function (event) {
    if (event.isApiReady) {
      this.initialize_app();
    }
  };

  HangoutBand.prototype.initialize_app = function () {
    audio = new WebAudio();

    if (audio.AUDIO_SUPPORT) {
      for (nr in sounds) {
        this.initialize_sound(nr);
      }

      // to all sorts of stuff to make the UI work

      // add callback when others send a "sound"
      window.gapi.hangout.data.onMessageReceived.add(this.onMessageReceived.bind(this));
    
    } else {
      // sorry no web audio api...
    }
  };

  HangoutBand.prototype.initialize_sound  = function (nr) {
    audio.load_sound(nr, function () {
      // sound finished loading
      window.document.getElementById(sounds[nr].div).onclick = function () {
        this.play_sound("", nr);
      }.bind(this);
    });
  };
  
  HangoutBand.prototype.onMessageReceived = function (event) {
    this.play_sound(event.senderId, event.message);
  };

  HangoutBand.prototype.play_sound = function (id, nr) {
    var chk_play = false;

    if (id && id != window.gapi.getParticipantId()) {
      // sound by someone else
      if (this.receiving) {
        chk_play = true;
      }
    } else {
      // your own sound
      chk_play = true;
      if (this.sending) {
        // send message about played sound to others
        window.gapi.hangout.data.sendMessage(nr);
      }
    }

    if (chk_play) {
      audio.play_sound(nr);
      // do all kinds of stuff related to recording the score and handling animations and stuff...
    }
  };

  hangout_band = new HangoutBand();

}(window));