HS125 FLC PIC BECOMES DISTR WITH COCKPIT DUTIES; ASSUMES HE IS CLRED TO TAXI TO ONE RWY WHEN CLRNC WAS TO ANOTHER. FLC TAXIES ACROSS RWY WITHOUT CLRNC WHILE PROCEEDING ENRTE FOR DEP.

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WE WERE GIVEN TAXI INSTRUCTIONS TO RWY 13 VIA TXWY C; HOLD SHORT OF RWY 22R. I BELIEVE I WAS DISTR SOMEWHAT DUE TO THE FACT I WAS USING A PART-TIME COPLT WHO WAS BEHIND THE ACFT; AND THAT I HEARD WHAT I HAD EXPECTED TO HEAR. I ASSUMED RWY 22 WAS ACTIVE. TAXI VIA TXWY C AND HOLD SHORT OF RWY 22 INSTRUCTIONS APPLY TO BOTH RWY 13 AND RWY 22 DEPS. WHILE TAXIING TO RWY 22; I CROSSED RWY 13/31 (AFTER HAVING MADE SURE IT WAS CLR). AT THAT TIME; GND TOLD US WE WERE GOING TO THE WRONG RWY. THEY WERE VERY COOPERATIVE; GAVE US A L TURN TO BACK TAXI TO RWY 13 WITH NO HARM BEING DONE. I BELIEVE I WAS DISTR WITH THE COPLT; AND DUE TO THE RUSH OF PAX ARRIVING 1 1/2 HRS EARLY WHILE TAXI INSTRUCTIONS WERE ISSUED; MY VOLUME WAS TURNED DOWN CAUSING MY MISUNDERSTANDING. I HAD HEARD THE RWY 22 BUT NOT RWY 13. THE ACTION OR PREVENTION SHOULD BE TO GUARD AGAINST HEARING WHAT YOU EXPECT TO HEAR; SLOW DOWN IF RUSHED OR DEALING WITH AN INEXPERIENCED CREW MEMBER; AND IF NOT SURE; THEN ASK FOR CLARIFICATION FROM CTL.