CPR Y NON ADHERENCE TO FAR CROSSED RWY WITHOUT CLRNC. RWY INCURSION.

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ON ARR AT THE SDF; CPR X LANDED NORMALLY ON THE ACTIVE RWY (RWY 19). ON ROLLOUT AND DURING THE DECELERATION; TWR ASKED WHICH FBO WE WERE GOING TO. I REPLIED; AND THEY (TWR) INSTRUCTED L TURN OFF THE RWY 19 ONTO THE TXWY. I HAD APPARENTLY MISUNDERSTOOD THE TWR INSTRUCTIONS (WE TURNED L ONTO THE TXWY AND CONTINUED TO TAXI TO THE FBO WITHOUT CONTACTING GND CTL). AS WE CONTINUED TAXIING WE CROSSED RWY 29 TO CONTINUE TO THE FBO. AFTER WE CROSSED RWY 29 TWR REPLIED CPR X CONTACT GND CTL. UPON CALLING GND; GND CTL ASKED WHY WE HAD NOT CONTACTED THEM WHILE TAXIING. I REPLIED WE FORGOT TO SWITCH FREQS. GND CTL ALSO TOLD US NEVER TO CROSS A RWY WITHOUT CALLING THEM. I HAD BELIEVED THAT WE HAD BEEN ISSUED TAXI INSTRUCTION BY SDF TWR AND THAT THEY WERE GOING TO SUPERVISE OUR TAXI TO THE FBO. SDF AT THAT TIME OF THE MORNING WAS SLOW (TFC-WISE) AND I THOUGHT THAT TWR WAS ALSO WORKING GND. THIS FREQUENTLY OCCURS ELSEWHERE. I NEVER HEARD TWR'S INSTRUCTIONS TO CALL GND CTL DUE TO HIGH COCKPIT NOISE LEVELS DURING DECELERATION. SUPPLEMENTAL INFO FROM ACN 251617: AFTER CLRING RWY 19 AT SDF; MYSELF AND THE CAPT BOTH THOUGHT WE HEARD A CLRNC TO MONITOR THE TWR AND TAXI TO THE RAMP. AFTER XING AN INTERSECTING RWY; WE WERE TOLD TO CONTACT GND AND WAS THEN INFORMED WE WERE NOT CLRED TO TAXI AND DEFINITELY WAS NOT CLRED TO CROSS ANY INTERSECTING RWYS. FACTORS CONTRIBUTING TO THE PROB WAS BAD AUDIO AND LACK OF ATTN AND NO READBACK OF THE CLRNC FROM TWR. I TRY TO MAKE IT A HABIT TO VERIFY CLRNCS WITH THE OTHER CREW MEN; BUT DUE TO LOW CONCENTRATION ON TAXIING THE ACFT; I FAILED TO DO SO.