MacOS Mojave 10.14 security and privacy protections now require the user to enable microphone access for True-Fi. After installing True-Fi on macOS Mojave (or running it for the first time after updating to Mojave) you will be prompted by macOS to allow True-Fi's access to the microphone. Please choose 'OK' to allow.
If you didn't get this pop-up window or you wish to change your permissions later, you can adjust your privacy settings manually by going to:
System Preferences → Security & Privacy → Privacy → Microphone → True-Fi
and clicking on the check box next to the True-Fi app (image below).
Please note that you need to 'click the lock to make changes' and enter your log-in details to adjust these settings.