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I have been using AVAudioEngine to take audio from the mic and send it out over a WebRTC connection. When I use the iPhone device mic, this seems to work as expected. But if I run the app with bluetooth headphones connected, the engine reports this error when trying to start: [avae]  AVAudioEngine.mm:160   Engine@0x2833e1790: could not initialize, error = -10868 [avae]  AVAEInternal.h:109   [AVAudioEngineGraph.mm:1397:Initialize: (err = AUGraphParser::InitializeActiveNodesInInputChain(ThisGraph, *GetInputNode())): error -10868 Error starting audio engine: The operation couldn’t be completed. (com.apple.coreaudio.avfaudio error -10868.) I see that Error code -10878 is: @constant kAudioUnitErr_FormatNotSupported Returned if an input or output format is not supported ... kAudioUnitErr_FormatNotSupported = -10868 but that doesn't seem like it can be quite correct. I know that the output format is supported because the same format works correctly when my headphones are not attached. And I am pretty sure that the input format is supported because I am able to simply hook up Headphones InputNode -> Mixer -> Headphones OutputNode and correctly hear the audio from the mic. So I can only assume that this means the format conversion is not supported. My Questions: Is this a bug? Is there any way I can work around this? Notes: My full audio graph looks like this, where all the "mixers" are just AVAudioMixerNodes: // InputNode (Mic)  -> Mic Mixer -\ // 																 >-> WebRTC Mixer -> Tap -> WebRTC Framework // AudioPlayer 1 -> Player Mixer  -/ // // AudioPlayer 2 -> Player Mixer -----> LocalOutputMixer -> OutputNode (Device Speakers/Headphones) but the issue still happens even if I simplify down to this: InputNode (Mic)  ->	Mixer -> Tap -> WebRTC Framework Specifically it happens when a single mixer node is connected with an input format and output format as follows: The input format is: (lldb) po audioEngine.inputNode.inputFormat(forBus: 0).streamDescription.pointee ▿ AudioStreamBasicDescription 	- mSampleRate : 16000.0 	- mFormatID : 1819304813 	- mFormatFlags : 41 	- mBytesPerPacket : 4 	- mFramesPerPacket : 1 	- mBytesPerFrame : 4 	- mChannelsPerFrame : 1 	- mBitsPerChannel : 32 	- mReserved : 0 The output format WebRTC expects is: ▿ AudioStreamBasicDescription 	- mSampleRate : 48000.0 	- mFormatID : 1819304813 	- mFormatFlags : 12 	- mBytesPerPacket : 2 	- mFramesPerPacket : 1 	- mBytesPerFrame : 2 	- mChannelsPerFrame : 1 	- mBitsPerChannel : 16 	- mReserved : 0 My headphones are Jaybird Freedom 2.