The 2.5mm switched jack socket I’m planning to fit to the end of the C3 case – probably where the LED is currently mounted.
I plan to cut the +ve wire on the internal microphone and route a wire to the jack socket and back. When the 2.5mm plug is inserted, it will break the return connection to the internal mic, and will instead connect the +ve lead from the external mic.
I’m planning to use the standard C3 external mic, so hopefully there should be no impedance mismatches. This will avoid the need to make a special combined power and external mic lead, and will also switch the internal mic off when the external mic is connected – if it works, that is!