/** Sets the playback speed. This method is executed on the caller's thread. */
 private void setSpeedSync(float speed) {
   playerLock.lock();
   if (media != null && media.getMediaType() == MediaType.AUDIO) {
     if (mediaPlayer.canSetSpeed()) {
       mediaPlayer.setPlaybackSpeed((float) speed);
       if (BuildConfig.DEBUG) Log.d(TAG, "Playback speed was set to " + speed);
       callback.playbackSpeedChanged(speed);
     }
   }
   playerLock.unlock();
 }