We did establish that it is working on my MBP using High Sierra, so it is related to the new permissioning in Mojave and newer.
This looks relevant:
default 13:27:27.862380 +0000 trustd cert[2]: AnchorTrusted =(leaf)[force]> 0
error 13:27:27.865835 +0000 tccd Prompting policy for hardened runtime; service: kTCCServiceMicrophone requires entitlement com.apple.security.device.audio-input but it is missing for RESP:{ID: com.dayturn.viewer, PID[16737], auid: 501, euid: 501, responsible path: '/Applications/Dayturn Viewer.app/Contents/MacOS/Dayturn', binary path: '/Applications/Dayturn Viewer.app/Contents/MacOS/Dayturn'}, ACC:{ID: ??, PID[16758], auid: 501, euid: 501, binary path: '/Applications/Dayturn Viewer.app/Contents/Resources/SLVoice'}, REQ:{ID: com.apple.tccd, PID[137], auid: 0, euid: 0, binary path: '/System/Library/PrivateFrameworks/TCC.framework/Versions/A/Resources/tccd'}
default 13:27:27.865887 +0000 tccd Policy disallows prompt for RESP:{ID: com.dayturn.viewer, PID[16737], auid: 501, euid: 501, responsible path: '/Applications/Dayturn Viewer.app/Contents/MacOS/Dayturn', binary path: '/Applications/Dayturn Viewer.app/Contents/MacOS/Dayturn'}, ACC:{ID: ??, PID[16758], auid: 501, euid: 501, binary path: '/Applications/Dayturn Viewer.app/Contents/Resources/SLVoice'}, REQ:{ID: com.apple.tccd, PID[137], auid: 0, euid: 0, binary path: '/System/Library/PrivateFrameworks/TCC.framework/Versions/A/Resources/tccd'}; access to kTCCServiceMicrophone denied
default 13:27:27.866452 +0000 kernel + IOAudioEngineUserClient::externalMethod, selector=0x0, arg0 0x0, arg1 0x101457000, arg2 0x8010 arg3 0x1e2