code: public void addSource(GVRAudioSource audioSource)
      {
          synchronized (mAudioSources)
          {
              if (!mAudioSources.contains(audioSource))
              {
                  audioSource.setListener(this);
                  mAudioSources.add(audioSource);
              }
          }
      }
docs: Adds an audio source to the audio manager.
  An audio source cannot be played unless it is
  added to the audio manager. A source cannot be
  added twice.
  @param audioSource audio source to add