void EngineOnJoinChannelSuccess (string channelName, uint uid, int elapsed) {
    string joinSuccessMessage = string.Format (
        "joinChannel callback channel {0}, uid: {1}, elapsed: {2}",
        channelName, Convert.ToString (uid), Convert.ToString (elapsed));
    Debug.Log (joinSuccessMessage);

    // show mute button
    muteButton.SetHidden (false);
}