GND INCIDENT OCCURS AS DH8 CAPT FAILS TO HOLD SHORT OF RWY BEING USED BY OTHER LNDG TFC. PLT DEV.

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I WAS JUMPSEATING ON FLT FROM MIA TO TPA. THERE WERE NO ADDITIONAL HEADSETS ON THE ACFT SO THE CAPT HAD A COCKPIT SPEAKER TURNED ON SO THAT I COULD MONITOR ATC AND FOLLOW ALONG WITH THE CREW'S COMS; AS WELL AS TFC WATCH. UNFORTUNATELY; THE SPEAKER WAS IMPOSSIBLE TO HEAR FROM THE JUMPSEAT; SO I MISSED MOST ATC XMISSIONS. ON THE APCH INTO TPA; WE WERE CLRED FOR A VISUAL TO RWY 27. I DIDN'T HEAR THE CLRNC; BUT IT WAS OBVIOUS. I DID NOT SPEAK TO THE CREW MEMBERS BELOW 10000 FT BECAUSE IT WAS STERILE PERIOD AND I COULD NOT FOLLOW THEIR ATC XMISSIONS. WE LANDED NORMALLY AND AFTER ROLLING THROUGH RWY 18L INTXN; WE TURNED OFF THE RWY. I NOTICED A FLURRY OF XMISSIONS WITH TPA TWR; AND AFTER WE GOT TO THE GATE; THE CAPT INFORMED ME THAT THE ACFT I HAD SEEN STOPPED ON RWY 18L HAD JUST LANDED ON THAT RWY AND WE WERE SUPPOSED TO HOLD SHORT OF RWY 18L. THE CAPT HAD NOT HEARD THE HOLD SHORT INSTRUCTION; ALTHOUGH THE FO SAID HE ACKNOWLEDGED THE CLRNC. I HAD HEARD NONE OF THE XMISSIONS DUE TO MY LACK OF HEADSET AND THE LESS THAN EFFECTIVE COCKPIT SPEAKER. IF I HAD BEEN ABLE TO MORE CLOSELY MONITOR THE ATC XMISSIONS; I MAY HAVE BEEN ABLE TO HELP IN THIS CASE. I WOULD LIKE TO POINT OUT THAT THIS CAPT GAVE ME A THOROUGH BRIEF PRIOR TO FLT AND SAID THAT HE CONSIDERED ME A PART OF THE CREW AND TO CALL OUT ANYTHING I FELT WAS UNUSUAL. THE LACK OF ADDITIONAL HEADSETS LEFT ME UNABLE TO.