B737-700 CREW HAD TKOF ABORT BECAUSE OF A TKOF WARNING HORN AT MDW.

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AFTER START CHKLIST COMPLETED; CALLED FOR TKOF FLAPS; TAXIED TO RWY 31C. COMPLETED PRETKOF CHKLIST; THE CLRED POS AND HOLD WITH PRETKOF CHKLIST ACCOMPLISHED WHEN CLRED FOR TKOF (FROM STANDING STILL); ADVANCED THROTTLES AND IMMEDIATELY GOT THE TKOF WARNING HORN WITH LESS THAN 5-10 KTS ON ACFT. THROTTLES IMMEDIATELY REDUCED AND WE TOLD TWR THAT WE 'NEEDED TO CLR THE RWY' AND PROCEED BACK TO THE HOLD SHORT. WE EXITED RWY 31L AT TXWY K AND RETURNED TO THE HOLD. WHEN WE GOT THE HORN; I IMMEDIATELY NOTICED THE FLAP HANDLE IN THE UP POS WITH THE LEADING EDGE DEVICES AND TOTAL TEF'S SHOWING NORMAL UP. THE TKOF WARNING HORN FUNCTIONED NORMALLY. WE SET THE FLAPS TO POS 5; RECOMPLETED PRETKOF CHKLIST IN ENTIRETY AND WAITED FOR ANOTHER CLRNC TO TAKE OFF. FLT CONTINUED NORMALLY AND WITHOUT FURTHER INCIDENT. THIS WAS DEFINITELY HUMAN FACTORS/PLT ERROR -- NOT MECHANICAL. WE BOTH REMEMBER CALLING FOR FLAPS ON THE CHKLIST AND THE PROPER RESPONSE WAS MADE. HOW THE FLAPS GOT BACK TO UP OR NEVER PLACED TO 5 IS NOT CLR TO US. WE DID EVERYTHING AS PRESCRIBED BY THE COMPANY AND IN A SLOW METHODICAL AND DELIBERATE MANNER. WE WERE NOT RUSHING OR IN A HURRY. I AM A NEW CAPT AND HE IS A NEW HIRE IN HIS FIRST MONTH AFTER IOE. WE WERE DOING OUR DEAD LEVEL BEST TO BE AS CAUTIOUS AS POSSIBLE. I WAS CERTAIN THE SPD BRAKE HANDLE WAS NOT THE CAUSE. LESSONS LEARNED FOR BOTH OF US ARE MANY. WE THOROUGHLY DEBRIEFED AND RESOLVED TO BE READY OR EXPECT THE UNEXPECTED. I BELIEVE OUR CHKLISTS/PROCS ARE SOUND. A NEW CAPT AND A NEW FO; IN THIS CASE; PROVED TO BE A WEAKNESS.