You say you wiggled wires etc and listened on a hand held. What was the result of the hand held test? could you hear your transmission clearly? - If you can hear your transmission on a hand held that is obviously close to the transmitting antenna then the issue is not loss of MIC Audio but a loss of range.
This means one of three things normally
1. Low output power from the transmitter (the radio)
2. A poor VSWR (antenna system quality if you like), this can be the antenna itself, of even the co-axial cable that runs from the radio to the antenna. Poor matching between the antenna and the radio is ensuring most of the power is being lost. A tech can test the VSWR from the radio tray and check this number for you if needs be, the antenna specification will be on the internet, but in essence your looking for less than 2:1. You can also use a thru line Wattmeter to measure power if needs be, clearly this should be close to the output power as specified by Narco for the unit.
Assuming you cannot hear your transmissions - see my post above
You have positions P1 and P2, is the fault apparent when you transmit from either side?
Can you hear yourself when transmitting (sidetone)? if you can then you can be assured your voice is at least reaching the intercom.
Let us know the answers to the above questions
regards
The Doc