Any way to make the Microphone on the DK Bongo controller function?

[shuffle2]

The controller itself internally maps the microphone amplitude to a trigger value (analog R trigger).
The GC to PC adapter doesn't really have knowledge of this, it's just passing what it thinks are gamepad controller inputs to the application.
Have you tried setting the device plugged into the port as bongos in the dolphin? Configure->Gamecube->Port->Bongos