SF340-A READBACK HEARBACK ERROR CAUSES LOSS OF SEPARATION.

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WE DEPARTED CRW AT XA24. THERE WERE NUMEROUS TSTMS IN THE AREA. TWR HAD GIVEN US A NORTHERLY HDG; BUT WITH A CLRNC TO DEVIATE AROUND WX AS NECESSARY. WE WERE IN IMC AND USING THE ACFT'S WX RADAR TO NAV AROUND STORM CELLS. DURING THE CLB; PASSING THROUGH 6500 FT; THE CAPT ASKED ME TO SPEAK WITH THE FLT ATTENDANT VIA THE INTERPHONE TO ENSURE THAT SHE REMAINED SEATED UNTIL FURTHER NOTICE DUE TO THE TURB. WHILE I WAS OFF FREQ; THE CAPT RECEIVED A CLRNC TO AN ASSIGNED ALT. HE DID NOT HEAR THE CTLR; AND ASKED THE CTLR TO REPEAT. WHEN I RETURNED TO COM #1; I HEARD THE CAPT READING BACK '8000 FT DIRECT PKB WHEN ABLE;' AND WATCHED AS HE DIALED 8000 FT INTO THE ALT SELECTOR. THE CTLR MADE NO RESPONSE TO MY CAPT'S READBACK; THUS I HAD NO REASON TO QUESTION THE CLRNC OR BELIEVE THAT THERE HAD BEEN A MISTAKE. AT 7300 FT; WE RECEIVED A TCASII ALERT THAT PROMPTED THE CAPT TO ASK FOR VERIFICATION OF THE ASSIGNED ALT OF 8000 FT. THE CTLR (DIFFERENT THAN THE PREVIOUS CTLR BECAUSE OF A CTLR CHANGE) SAID 'NEGATIVE; DSND AND MAINTAIN 7000 FT.' EVEN AS THE CTLR WAS SPEAKING; WE WERE IN OUR DSCNT BECAUSE THE TA HAD BECOME AN RA; COMMANDING US TO DSND. THE RA OCCURRED AT 7500 FT; AND WE WERE CLR OF CONFLICT AT 7000 FT. THE CTLR NOTIFIED US THAT THERE HAD BEEN A POSSIBLE PLTDEV AND ASKED US TO CALL THE CRW TWR WHEN WE REACHED OUR DEST. THERE WERE SEVERAL CONTRIBUTING FACTORS INVOLVED. BECAUSE OF THE SEVERE WX IN THE AREA; THERE WERE MULTIPLE ACFT DEVIATING TO FIND A CLR FLT PATH. THE EFFECT IS MORE ACFT AND LESS USABLE AIRSPACE. SECOND; THERE WAS NO REDUNDANCY IN COCKPIT RADIO COMS BECAUSE I WAS USING THE INTERPHONE TO TELL THE FLT ATTENDANT TO REMAIN SEATED FOR HER SAFETY. ONE CANNOT HEAR RADIO COMS WHEN TALKING ON THE INTERPHONE. THIRD; THERE WAS A BREAKDOWN IN PLT/CTLR RADIO COMS. THE CAPT DID NOT HEAR THE FIRST ASSIGNED CLRNC FROM THE CTLR AND WHEN HE ASKED HIM TO REPEAT IT; THE CTLR READ BACK A HURRIED CLRNC. THE CAPT READ BACK WHAT HE THOUGHT HE HEARD -- 8000 FT -- AND THE CTLR DID NOT CORRECT HIM. WE LATER FOUND OUT THERE WAS A CTLR CHANGE OCCURRING AT THAT TIME WHICH IS A POSSIBILITY FOR WHY THE BREAKDOWN OCCURRED. HAD IT NOT BEEN FOR TCASII ALERTING US; WE WOULD NOT HAVE QUESTIONED THE ALT. THOUGH WE WERE PASSING THROUGH 7300 FT; THE CTLR DID NOT INITIATE RADIO CONTACT WITH US. THERE ARE SEVERAL THOUGHTS TO KEEP THIS SIT FROM OCCURRING AGAIN. WHEN ONE PLT IS OFF FREQ AND A CLRNC IS RECEIVED BY THE OTHER PLT; THE CLRNC SHOULD BE VERIFIED BY THE PLT RETURNING TO THE FREQ. DURING TIMES OF SEVERE WX AND DEVS; EXTRA VIGILANCE SHOULD BE TAKEN BY THE CTLRS WHEN CHANGING STATIONS OR RECEIVING ONE ANOTHER. SOME DAY I WOULD LIKE TO SEE SOMETHING LIKE ACARS FOR ATC COMS. THAT WAY; PLTS COULD LOOK AT A COMPUTER SCREEN AND VERIFY INSTRUCTIONS VISUALLY AS WELL AS AUDIBLY.