The latest version of OSX Mojave you need to allow Hindenburg to use the microphone. 

Even if you are not using the built in microphone, you still need to allow it to be able to record. 

Try the following: 

Open Apple / System Preferences.

Open Security & Privacy

Choose the "Privacy" pane at the top

Choose Microphone

Scroll down to Hindenburg and make sure it's ticked.