I know that some Windows (mostly OEM) distros come with a driver(?) that allows you to transmit through your mic from whatever is being transmitted through your soundcard.

My friend had an option in his advanced Sound options that allowed him to do it, but mine lacked it.

Do any of you know where I can maybe download the driver/module for it?

You mean use your microphone as a speaker? Or your speakers as microphones (so to speak)?

I know nearly all drivers should support handling a "Stereo/Mono Mix" to feed into an "Input" setting (basically making any outgoing sounds transmit through a Line In or Mic input).

My audio settings do not support this, but many programs I use have settings for it. I guess it depends on what additional software comes with your sound card or the programs you use.