The CVR power logic is controlled by relays which in turn depend on engine low oil pressure switches and landing gear compressed or not compressed sensing switches connected to LGCIUs(Landing gear interface control unit.
If these relays are not operating correctly , you can get this type of problem.
What you hear from CVR jack on overhead panel is what is actually being recorded after delay of about half a second.This is possible if your CVR is powered .
In the scenario you described it could be that you might have plugged in CVR jack within 5 minutes of last engine shut down or within first 5 minutes of application of external power to aircraft electrical network.
other possibility is defective relays.This not desirable as the same set of relays are also controlling the flight data recorder and there is a possibility of losing valuable recording when it is not intended to be powered.
You can write your observation in the tech log,maintenance engineer will be able to fix the problem.