Detecting faults in CVR mic inputs
Interesting question. On the Boeing 727 there is a panel named "Cockpit Voice Recorder Microphone Monitor". A headphone can be plugged into the labelled jack to test if the cockpit voice recorder microphone is working properly. I have not checked the other model aircraft you mentioned but I assume they have the same test capability.



These microphones are usually passive devices. I doubt there is a warning system even on modern airliners to tell you the CVR mike is not working. But I would also be willing to bet good money they are no longer so easy to disable.