A B737-300 CAPT RPTS OLDER -300 ACFT ARE ALLOWING 'BLEED THROUGH' OF LOUD RADIO MUSIC IN HEADSETS WHEN VOR RECEIVERS TUNED TO 108 PT 0 AND 115 PT 9 MHZ.

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I WAS FLYING THE TRAGR ARR INTO LAS. THE VOR/ILS/NAV SWITCHES ON THE -300 WE WERE FLYING WERE BOTH DOWN FOR THE RNAV ARR PROC. BOTH RADIOS WERE IN AUTO AND HAD DIALED 108.0 MHZ IN THEIR RESPECTIVE WINDOWS; WHICH MEANS THEY WERE BOTH 'AGILITY TUNING' FOR MAX EFFECTIVENESS TO ASCERTAIN OUR POS. WHILE THE RADIOS WERE USING 108.0 MHZ; BOTH OF US HAD LOUD RADIO MUSIC PLAYING IN OUR HEADSETS. THERE WERE NO OTHER RADIOS SELECTED ON EITHER OF OUR AUDIO SELECTOR PANELS AND THE VOLUME ON ALL RADIOS WAS TURNED ALL THE WAY TO MINIMUM (I CHKED THEM ALL). WE COULD NOT GET RID OF THE RADIO MUSIC AND IT WAS LOUD ENOUGH TO BE VERY DISTRACTING AND MADE HEARING ATC COMS DIFFICULT. I SPOKE TO MECHS ABOUT THE SIT IN LAS AND WAS INFORMED THAT THEY COULD ONLY REPLACE 'LINE ITEMS' AND THAT WHAT I WAS DESCRIBING WAS A KNOWN PROB WITH THE OLDER -300 AIRFRAMES. IN FACT; WHILE I WAS TALKING TO THEM; THEY GOT A CALL FOR THE SAME PROB FROM THE ACFT NEXT DOOR TO OURS. I ELECTED TO NOT WRITE UP THE PROB GIVEN THAT I HAVE HAD THIS SIT BEFORE; AND THERE IS NOTHING THE LINE MECHS CAN DO TO AFFECT A SOLUTION EXCEPT CHANGE OUT RADIOS OR AUDIO SELECTOR PANELS. IN MY EXPERIENCE THAT SELDOM CLRS THE PROB; IT ONLY CLRS THE LOGBOOK ENTRY. THE PROB WE EXPERIENCED IS NOT ONLY FOUND IN LAS. WE HAD THE SAME PROB ON THE SUBSEQUENT FLT FROM LAS TO ELP; BOTH ON FREQ 108.0 AND 115.9 OVER ARIZONA. IN ADDITION; THE RADIOS ON THESE OLDER -300'S CANNOT BE DETUNED SO I DO NOT HAVE A LISTEN TO NAVAID IDENTS AND ATIS OR TRANSCRIBED WX BROADCASTS OVER THE VOR FREQS. I HAVE IDENTED TO MAINT SEVERAL ACFT IN THE PAST 2 MONTHS THAT HAVE RADIO PROBS OF THIS TYPE. ADDITIONALLY; I HAVE IDENTED AT LEAST 1 OTHER ACFT TO THEM; WHICH WAS IN FEBRUARY OF THIS YR; I BELIEVE. I SPENT 4 YRS AS AN AVIONICS TECHNICIAN IN THE US ARMY. I BELIEVE THE PROBS WE ARE EXPERIENCING WITH THE OLDER -300'S ARE A COMBINATION OF INSTALLATION AND CALIBRATION ERRORS THAT HAVE CREPT IN OVER THE YRS. THE EMPLOYEE I SPOKE WITH IS OF THE OPINION THAT MOST OF THE PROBS ARE DUE TO INSUFFICIENT OR MISALIGNED SHIELDING OF THE WIRES THAT CARRY THE SIGNALS FROM THE RADIOS TO THE OLDER STYLE AUDIO SELECTOR PANELS FOUND IN THE OLDER -300'S. I TOO BELIEVE THAT IS PART OF THE PROB; THOUGH NOT THE ENTIRE PROB. MOST OF THE OLDER STYLE AUDIO SELECTOR PANELS SEEM TO HAVE THE AUDIO GAIN SELECTED TO A VERY HIGH LEVEL; WHICH COULD CONTRIBUTE TO THE PROB. BY WAY OF EXPLANATION; EVEN WITH THE #2 RADIO SELECTOR VOLUME KNOW TURNED TO FULL 'OFF' POS; I CANNOT DESELECT THE COMPANY FREQ UNLESS I PRESS THE KNOB AND COMPLETELY DESELECT THE RADIO -- AND THAT DOES NOT WORK ON MANY OF THE OLDER AIRPLANES. IN MY OPINION; THIS CONSTITUTES A SAFETY PROB. LISTENING TO NAVAID IDENTIFIERS AND TRANSCRIBED WX BROADCASTS I CANNOT DESELECT FOR 10-12 HRS PER DAY IS FATIGUING. I DO NOT NEED THE ADDITIONAL NOISE IN OUR ALREADY LOUD COCKPITS. I HAVE BEEN FLYING THE B737-300 FOR 17 YRS AND THE AIRPLANES DID NOT EXHIBIT THIS PROB WHEN NEW; AT LEAST TO MY MEMORY. I DO NOT BELIEVE THEY SHOULD EXHIBIT THE PROBS NOW; AND WOULD NOT; IF THE RADIOS AND AUDIO SELECTOR PANELS WERE PROPERLY CALIBRATED AND SHIELDED.