WHILE HOLDING SHORT OF RWY 25 TO CROSS; THE PLT OF A C172 MISHEARS A XMISSION ON THE TWR FREQ AND WRONGLY BELIEVES HE IS CLRED TO CROSS.

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I HAD BEEN INSTRUCTED TO HOLD SHORT OF RWY 25 AT TXWY B AND WAS DOING SO. AFTER SEVERAL MINS OF HEARING NOTHING AND NOTICING ANOTHER ACFT HOLDING POS ON RWY 25; I BEGAN TO CONTACT GND TO REMIND THEM I WAS HOLDING SHORT. AT THAT INSTANT; I WAS CONTACTED ON TWR FREQ AND TOLD (MY UNDERSTANDING; WHICH TURNED OUT TO BE WRONG) THAT I WAS TO CROSS RWY 25. AS I BEGAN TO CROSS; THE CTLR TOLD ME TO STOP AND THEN DECIDED I SHOULD CONTINUE XING SINCE I WAS ALREADY ONTO RWY 25. AT THAT POINT; I WAS INSTRUCTED TO CONTINUE TO MY ASSIGNED RWY AND TO CALL TWR UPON MY RETURN TO THE ARPT. UPON ANALYSIS AND DISCUSSION WITH THE TWR THE EVENTS THAT LED TO THIS INCIDENT WERE: BOTH GND AND TWR WERE BEING MONITORED IN THE AIRPLANE; BUT THE VOLUME ON THE RADIO TUNED TO GND WAS TOO LOW TO HEAR WELL AND AS A RESULT; I MISSED A GND CLRNC TO CROSS RWY 25. SINCE I HAD BEEN CLRED TO CROSS RWY 25 AND DID NOT ACKNOWLEDGE OR CROSS; THE CTLR KEPT THE RWY 25 TFC IN A POS AND HOLD WAITING FOR ME TO CLR. DURING THIS TIME; THE CTLRS TRIED OTHER FREQS TO CONTACT ME AND FINALLY DID SO ON TWR FREQ. WHEN I HEARD THE CALL ON TWR; I MISUNDERSTOOD IT TO BE A CLRNC TO CROSS RWY 25; THEN I IMMEDIATELY SWITCHED THE RADIO TO GND REALIZING THAT I SHOULD BE WITH THEM AND NOT TWR AND ACKNOWLEDGED THE XING INSTRUCTION. THIS LED TO MORE CONFUSION AS THE GND CTLR HAD NOT PREVIOUSLY HEARD THE TWR'S XMISSION TO ME AND SO WAS NOT EXPECTING ANY ACKNOWLEDGEMENT. I REALIZE THAT THIS WHOLE INCIDENT WAS MY FAULT AND THAT SEVERAL MISTAKES WERE MADE. 1) MISSING THE GND CLRNC TO CROSS RWY 25. 2) NOT CLRLY UNDERSTANDING THE INSTRUCTION GIVEN TO ME ON THE TWR FREQ. 3) SWITCHING BACK TO GND TOO QUICKLY AFTER I HAD BEEN CONTACTED ON THE TWR FREQ. 4) INITIATING A RWY XING TOO HASTILY BEFORE I WAS SURE OF THE INSTRUCTION. LUCKILY; THE CTLRS WERE ON THE BALL AND WERE NOT GOING TO RELEASE THE OTHER ACFT UNTIL MY STATUS HAD BEEN RESOLVED. THIS STEP INSURED THAT AN ACCIDENT WOULD BE AVOIDED AND I AM GRATEFUL FOR THEIR PROFESSIONALISM. IN THE FUTURE; I WILL PAY EXTRA ATTN TO WHERE MY RADIOS ARE TUNED AND VERIFY ALL INSTRUCTIONS/CLRNCS.