RV-8 intercom fault

[Barry G]

Right ! Here's one for you comms guys from the land of song - Wales!

I have an RV8 with a Flightcom 403mc intercom and ATR 500 radio. It all worked fine until a few weeks ago when, suddenly, the front seat can't hear the rear seat on the intercom.
The aircraft was built to the Vans wiring diagrams and has worked OK for several years.

Front seat radio works fine and the rear seat can always hear the front seat OK.
Rear seat can transmit (and receive) on the radio OK and - when the rear seat transmits on the radio - the front seat can hear him OK.

All the jacks and cabling seem OK and we've tried numerous different headsets both front and back. Doesn't matter where you put the ISO / ALL switch on the 403mc or where you put the volume or squelch knobs.

Is it an internal intercom fault ?

All comments gratefully received. Barry G