title: Mix two AUNodes in Spotify iOS SDK
body: <p>I'm trying to use the dynamic processor and a bunch of filters to compress a specific frequency band within the spotify method <code>connectOutputBus</code> but when i mix the nodes in the <code>kAudioUnitSubType_MultiChannelMixer</code> only the sound of the first added node comes out. </p>
  
  <p>obs: i actually use filters on <code>sourceNodeCopy</code> to remove the freqs that will be compressed on the <code>souceNode</code> but to keep things short i omitted them.</p>
  
  <p>Here's the code:</p>
  
  <pre><code>override func connectOutputBus(_ sourceOutputBusNumber: UInt32, ofNode  sourceNode: AUNode, toInputBus destinationInputBusNumber: UInt32, ofNode destinationNode: AUNode, in graph: AUGraph!) throws {
  
    let sourceNodeCopy = sourceNode //original node without the harsh freq
  
    //create a filter for the harsh frequencies
    var filterDescription = AudioComponentDescription()
    filterDescription.componentType = kAudioUnitType_Effect
    filterDescription.componentSubType = kAudioUnitSubType_BandPassFilter
    filterDescription.componentManufacturer = kAudioUnitManufacturer_Apple
    filterDescription.componentFlags = 0
    filterDescription.componentFlagsMask = 0
  
    AUGraphAddNode(graph, &amp;filterDescription, &amp;filterNode!) // Add the filter node
    AUGraphNodeInfo(graph, filterNode!, nil, &amp;filterUnit!) // Get the Audio Unit from the node
    AudioUnitInitialize(filterUnit!) // Initialize the audio unit
    // Set filter params
    AudioUnitSetParameter(filterUnit!, kBandpassParam_CenterFrequency, kAudioUnitScope_Global, 0, 10038, 0)
  
    //create a processor to compress the frequency
    var dynamicProcessorDescription = AudioComponentDescription()
    dynamicProcessorDescription.componentType = kAudioUnitType_Effect
    dynamicProcessorDescription.componentSubType = kAudioUnitSubType_DynamicsProcessor
    dynamicProcessorDescription.componentManufacturer = kAudioUnitManufacturer_Apple
    dynamicProcessorDescription.componentFlags = 0
    dynamicProcessorDescription.componentFlagsMask = 0
  
    // Add the dynamic processor node
    AUGraphAddNode(graph, &amp;dynamicProcessorDescription, &amp;dynamicProcessorNode)
    AUGraphNodeInfo(graph, dynamicProcessorNode, nil, &amp;dynamicProcessorUnit)
    AudioUnitInitialize(dynamicProcessorUnit!)
  
    // Set compressor params
    AudioUnitSetParameter(dynamicProcessorUnit!, kDynamicsProcessorParam_Threshold, kAudioUnitScope_Global, 0, -35, 0)
    AudioUnitSetParameter(dynamicProcessorUnit!, kDynamicsProcessorParam_AttackTime, kAudioUnitScope_Global, 0, 0.02, 0)
    AudioUnitSetParameter(dynamicProcessorUnit!, kDynamicsProcessorParam_ReleaseTime, kAudioUnitScope_Global, 0, 0.04, 0)
    AudioUnitSetParameter(dynamicProcessorUnit!, kDynamicsProcessorParam_HeadRoom, kAudioUnitScope_Global, 0, 0, 0)
  
    //mixer
    var mixerDescription = AudioComponentDescription()
    mixerDescription.componentType = kAudioUnitType_Mixer
    mixerDescription.componentSubType = kAudioUnitSubType_MultiChannelMixer
    mixerDescription.componentManufacturer = kAudioUnitManufacturer_Apple
    mixerDescription.componentFlags = 0
    mixerDescription.componentFlagsMask = 0
  
    AUGraphAddNode(graph, &amp;mixerDescription, &amp;audioEffectsController.mixerNode)
    AUGraphNodeInfo(graph, audioEffectsController.mixerNode, nil,
                      &amp;audioEffectsController.mixerUnit)
    AudioUnitInitialize(audioEffectsController.mixerUnit!)
  
    AudioUnitSetParameter(mixerUnit!, kMultiChannelMixerParam_Volume, kAudioUnitScope_Input, 0, 1.0, 0);
      AudioUnitSetParameter(mixerUnit!, kMultiChannelMixerParam_Volume, kAudioUnitScope_Output, 0, 1.0, 0);
  
  
    //connect the nodes
    AUGraphConnectNodeInput(graph, sourceNode, sourceOutputBusNumber, filterNode, 0)
    AUGraphConnectNodeInput(graph, filterNode, sourceOutputBusNumber, dynamicProcessorNode, 0)
  
    AUGraphConnectNodeInput(graph, sourceNodeCopy, sourceOutputBusNumber, mixerNode, 0)
    AUGraphConnectNodeInput(graph, fiterNode, sourceOutputBusNumber, mixerNode, 0)
  
    //connect the mixer to the output
    AUGraphConnectNodeInput(graph, audioEffectsController.mixerNode, 0, destinationNode, destinationInputBusNumber)
  </code></pre>