[abrigham]

Yeah, I'm pretty sure it's hardware based. If you're using the 3-in-1 adapter then that's definitely what's triggering it. It is odd that if you plug the cassette adapter into the headphone jack first it doesn't cut off the mic - must have to do with the way the circuit is wired. But, yeah, you should be able to find a pin-out diagram for the extUSB port and pull or cover the mic-in pin - that would prevent it from closing the circuit that turns off the internal mic.