AN SMA PLT LANDED ENRTE WHEN HE CAME UPON SOME WX.

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I WAS VFR ON TOP AT 9500 FT. THE LAYER UNDER ME WAS BROKEN. I WAS ABOUT 25 MINS INTO MY FLT FROM MFR; OR; TO HQM; WA -- A 2 HR FLT. TRIED TO CONTACT FSS AND ZSE FOR WX AND FLT FOLLOWING; WITH A REPLY OF 'ACFT CALLING FSS/ZSE YOU'RE UNREADABLE.' MY RADIOS WERE XMITTING BUT MY VOICE WAS UNREADABLE. AT THAT POINT I DID NOT WANT TO CONTINUE VFR ON TOP WITH NO RADIOS. I STARTED SQUAWKING 7600 AND MY DSCNT TO THE NEAREST ARPT WITH SVCS; NORTH BEND; OR (OTH). STAYING VFR; I DSNDED TO 700 FT OVER SHORELINE ABOUT 5-7 MI FROM NORTH BEND ARPT. I WAS ABLE TO TALK WITH NORTH BEND UNICOM 122.95 ABOUT 10 MI OUT AND THEY WERE ABLE TO UNDERSTAND ME. I STAYED WITH 122.95 AND ASKED FOR SVFR; DENIED. I DID NOT WANT TO CONTINUE IN THE MARGINAL CONDITIONS. UNICOM ASKED IF I WANTED TO DECLARE AN EMER TO LAND. I RESPONDED YES. STAYED CLR OF THE CTL ZONE 5 MI SW OVER THE COAST LINE UNTIL CLRNC WAS GIVEN TO ENTER THE CTL ZONE. I WAS ABLE TO MAINTAIN VFR AND NAV VISUALLY TO THE ARPT AND LAND. CONTACTED ZSE BY LANDLINE/TELEPHONE. FOUND MY MICROPHONE TO BE BAD. MY DECISIONS WERE MADE EARLY BEFORE ANY STRESS STARTED. THE EMER WAS A PRECAUTIONARY MEASURE TAKEN BY ME.