well if plugging the disabled 3rd party mic somehow leaves the internal one recording, that would imply your soundcard just got upgraded to having the ability for two inputs recording simulataneously. usually cost a premium, such cards :)
Unfortunately, input and output switching is usually implemented in software these days, so an attacker could probably just remotely switch to the internal mic on most laptops.
I also wonder if plugging a 3rd party mic into line-in and switching it off would have a similar effect for the mic.
You can never be sure unless your fix is physical.