The student had recorded her primary flight lesson on a tape recorder. After reaching the practice area, the CFI instructed the student to turn 180 deg left. The student complied and performed a shallow bank turn. Seconds later, while cruising at 2,200 ft, the CFI began talking. In midsentence an undetermined event occurred which interrupted his speech. A wind-like background noise started, and the student exclaimed 'help.' Radar data confirmed witness reports that the tail boom and M/R had separated in level flight. Examination of the wreckage indicated mast bumping had occurred, a M/R blade crushed the left side of the inboard tail cone, and the entire mast with attached M/R blades broke out of the top of the transmission. The outboard tail boom and rotor assembly have not been found. Evidence of a main rotor blade divergence was found. Spectrum analysis of the tape revealed no anomalies until the CFI's voice was terminated. Within 2 seconds the rotor speed appeared to significantly oscillate and abruptly end.