In the VO window you have to select the audio input which could be from the built-in mic, Line In or USB etc.
It's possible that this setting has been altered accidentally.
Also open the Sound pane in System Preferences and check that your input parameters are all correctly set.
If everything appears to be set correctly, there may be a glitch in FCP, so trash all the preferences using this free app
I am running blackmagic and all my inputs and settings are blackmagic are set to black magic.
Tried your app , but still no luck...is it possible there is a problem with the blackmagic card in my mac?