FALCON 20 CREW LOST COMS WITH APCH CTLR FOR SHORT TIME DUE TO INADVERTENT TURN DOWN OF THE COM RADIO VOLUME CTL.

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AT THE TIME OF THIS EVENT I WAS FO (PNF) OF A FALCON ACFT ENRTE TO IAD ON THE COATT 3 ARR. WE WERE ESTABLISHED ON THE LOC FOR RWY 1R AT 5000 FT MSL; APPROX 25 NM S OF THE RWY. WE WERE IMC; IN TURB. AFTER CONTACTING DULLES APCH CTL AND BEING ASSIGNED THE ILS RWY 1R; I REQUESTED RWY 1L; AS OUR FBO OF CHOICE IS LOCATED ON THE NW SIDE OF THE ARPT. THE CTLR ADVISED US THAT HE WOULD CHK TO SEE IF THIS COULD BE WORKED OUT. SINCE THERE ARE 2 DIFFERENT TWR FREQS; ONE FOR EACH RWY; I REPROGRAMMED THE RWY 1L TWR FREQ INTO THE COLLINS CTL-22 CTL HEAD OF OUR RADIO IN USE. BECAUSE OF THE TURB; MY HAND INADVERTENTLY HIT THE VOLUME CTL AND LOWERED THE VOLUME TO AN INAUDIBLE LEVEL WITHOUT DETECTION BY THE CREW. AFTER ANOTHER FEW MINS OF FLYING IN THIS TURB AT 5000 FT MSL; I REQUESTED A LOWER ATL FOR A BETTER RIDE.; AFTER HEARING NO RESPONSE AND NOTHING BUT SILENCE; THE CAPT (PF) TURNED THE SQUELCH CTL AND FOUND THE VOLUME TURNED TOO LOW TO BE AUDIBLE. WE INCREASED THE VOLUME AND RE-ESTABLISHED COM WITH APCH CTL. THE CTLR TOLD US HE HAD ATTEMPTED TO CONTACT US SEVERAL TIMES WITHOUT SUCCESS. HE HAD INDEED OBTAINED CLRNC FOR US TO LAND ON RWY 1L; BUT WITH THE DELAY IN COM; OUR REQUEST WAS NOW DENIED. WE WERE CLRED FOR RWY 1R; AND TO CONTACT THE TWR. WE CONTACTED THE TWR; FLEW THE APPROPRIATE APCH; AND LANDED WITHOUT INCIDENT. AT THE RAMP WE WERE INSTRUCTED TO CALL APCH CTL VIA TELEPHONE AND THE PF DID SO AND EXPLAINED THE PROB WE HAD TO HIM. WE ACKNOWLEDGED THE PROB AND APOLOGIZED FOR ANY PROBS THAT THIS MIGHT HAVE CAUSED. FLCS USING THIS PARTICULAR COM CTL HEAD SHOULD BEWARE OF THE POTENTIAL PROB AND MAINTAIN EXTRA CARE TO PRECLUDE THIS FROM HAPPENING TO THEM. WE NOW DOUBLECHK THE VOLUME CTL AFTER EACH FREQ CHANGE.