Resolved.
I needed to add an entitlement in Xcode allowing the application to use the (virtual) microphone.
geoffw2802
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More info... I'm using a Mac Mini which does not have a built-in microphone.
However, Audio MIDi happily allows BlackHole to be Used for Sound Input in Audio MIDI.
Also, I get this message in the Xcode console when my app first starts: AddInstanceForFactory: No factory registered for id <CFUUID 0x600000970380> F8BB1C28-BAE8-11D6-9C31-00039315CD46
Google doesn't turn up anything suggesting this is causing the issue I'm seeing though. Seems to be related to Xcode.