I have struck an unserviceability with the Cockpit Area Microphone (CAM) on several occasions with both the B737-200 and the B737 Classics. The CVR test was normal (press the test button and note status light and tone). But when a headset is plugged into the headset jack and you say a few words out aloud, or jiggle the flap lever to hear the noise through the CAM, either you didn't hear a thing, or it was too muffled to pick up words. Either way it would have been useless as far as listening to crew conversation or specific noises in the cockpit.
I would write always up the defect in the maintenance log. A specific check of the CAM serviceability is not a part of the Boeing before start scan nor is it called for in the amplified procedures and for that reason I would not be at all surprised if unserviceable CAM's are rarely picked up. I made a point of checking the CAM serviceability on every flight - it's dead easy to check it out.