Cordova - Control several sounds on Android

I manage to play several sounds on Android (Cordova 4.1) but now I would like to dynamically change the volume of each video.

My code for playing sounds is the following:

function onDeviceReady() {
  document.querySelector("#play").addEventListener("touchend", playMP3(), false);

  function playMP3() {
        var mp3_1 = getMediaURL("audio/sound1.mp3");
        var mp3_2 = getMediaURL("audio/sound2.mp3");
        media1 = new Media(mp3_1, null, mediaError);
        media2 = new Media(mp3_2, null, mediaError);
        media1.play();
        media2.play();
  }

  function getMediaURL(s) {
      if(device.platform.toLowerCase() === "android") return "/android_asset/www/" + s;
      return s;
   }
  function mediaError(e) {
      alert('Media Error');
      alert(JSON.stringify(e));
  }
}

Then I would like to control volume with something like media1.volume = 0.5; from outside the onDeviceReady function but I can't make it work...

Any idea of what I'm missing?