Mac defaults to device 0 for new Audio Mixer

Users are pointing out issues with the new Audio Mixer on Macs. We have already responded on the forum post and are aware of its current state.

One user has identified the issue with "default macOS audio device not being used is caused by InitializeHardware in AudioMixerDevice.cpp. It explicitly sets the device array index to be 0 (unless DefaultDeviceName is set or VR audio is being used). The default device does seem to be 0 on windows but this is not the case on the mac, and this function isnt making use of DefaultDeviceIndex which is set in the core audio-specific code.

This problem didn't show up in windows because it seems the default audio device is always first in the array on that platform and all the GetDefaultOutputDeviceIndex is doing on windows is returning 0 anyway, but this is not so for every platform."

Launch UE4 Editor with the -audiomixer command line option on MacOS.

Expected Result:
Editor's audio output outputs to the audio device specified as the default device in system preferences

Actual Result:
Editor will always output audio to the audio device at index 0, almost always the built-in output.