Playing AAC multichannel (5.1 surround) audio stream on Apple TV

I have written a tvOS app which receives AAC 5.1 audio stream over the network and I am using AudioToolbox to decode (mFormatID = kAudioFormatMPEGAAC) to 5.1 channels. But I only hear 2 channels (Stereo) outputs on my 5.1 surround system (which is connected to the TV using an Audio Optical cable).

I am not sure why PCM multi-channel is not outputting from AppleTV. Am I missing anything?

Decoder parameters:

    AudioStreamBasicDescription asbd
= {0};

    asbd.mFormatID =
kAudioFormatMPEG4AAC;

    asbd.mFormatFlags =
kMPEG4Object_AAC_Main;

    asbd.mSampleRate = samplingRate;

    asbd.mBitsPerChannel = 0;

    asbd.mBytesPerPacket = 0; 

    asbd.mFramesPerPacket = 1024;

    asbd.mBytesPerFrame = 0;。

    asbd.mChannelsPerFrame = numOfChannels; // 6

    asbd.mBitsPerChannel = 0; 

    asbd.mReserved = 0; 

Additionally, when I query the total number of audio channels on AppleTV, it is always 2 (even though it is connected to the surround system). This is just for me to know audio capabilities.

AVAudioSession.sharedInstance().currentRoute.outputs[0].channels