B737-300 FLT CREW HAS A REJECTED TKOF AT OKC.

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NORMAL PREFLT; BRIEF AND STARTUP FOR DEP. CAPT'S LEG WAS PF. BEGAN TKOF ROLL ON RWY 17R AT OKC. AT AROUND 50 KIAS; A 'WHISTLING/SCREECHING' SOUND WAS NOTICED IN THE COCKPIT. INITIALLY; CREW THOUGHT IT WAS DUE TO POSSIBLE PRESSURIZATION SEALING AT HIGH PWR. GOING THROUGH 80 KIAS; NOISE BECAME INCREASINGLY LOUDER AND SOUNDED LIKE SOMETHING WAS GRINDING. BTWN 80-100 KIAS CAPT INITIATED REJECTED TKOF MANEUVER. MANEUVER WAS PERFORMED PERFECTLY WITH ACFT ON CTRLINE; CALL TO FLT ATTENDANTS TO REMAIN SEATED; AND TWR TO ROLL THE TRUCKS FOR BRAKE INSPECTION. SUBSEQUENTLY; CREW CANCELLED REQUEST FOR EMER VEHICLES AS THERE WERE NO OTHER INDICATIONS OF A PROB; AND THE NOISE SUBSIDED. TAXIED BACK TO THE RAMP; DEPLANED; AND CONTACTED MAINT CTL. EXTERNAL INSPECTION OF THE ACFT PROVIDED NO INDICATIONS OF PROB. JUMPSEATING FLT ATTENDANT SUGGESTED THAT IT SOUNDED LIKE AN AUDIO SYS MALFUNCTION -- WHICH IS WHAT IT ENDED UP BEING. BOARDING MUSIC SYS HAD INADVERTENTLY SWITCHED ON WITH FULL VOLUME AND ALMOST UNBEARABLE 'HISS/SCREECH' IN THE CABIN. MAINT DEFERRED THE SYS AND FLT PROCEEDED WITHOUT FURTHER INCIDENT.