There is an interesting fact about Headphones i.e. they can also be used as a microphone !
There is not much difference between the circuits of a Microphone and a Headphone , just the headphone is designed to spread voice and an microphone to capture the voice !
So if you connect the headphone in the microphone slot of your computer, you could use it as a microphone.
Just speak in the ear buds of the microphone, but please speak a bit loudly to be audible at the other end !
** Remember it is just a temporary solution !