Speed of sound through the aircraft.
Wouldn't an high explosive event in the tail likely sever the electrical connections to the CVR before the sound could travel through the air to the microphones in the cockpit?
The speed of sound in Aluminum is about 15 times faster than the speed of sound in air. An explosive event would be rich in high frequency components and would travel through the structure and in effect, shake the structure holding the microphones and generate a signal in that manner. There would be a number of potential methods of analyzing the signals from the various cockpit microphones to confirm an explosive event.
If the event acoustic signal in the cockpit is an air carried signal, then the likelihood of a structural failure cause for the sound would be very high.