In my alienware laptop its called "rec playback" and not "stereo mix". i made that the default recording device.
that setting though only records what is played back and didnt pick up the mic. to pick up the mic, on the same recording tab, go to the properties of the device you normally have, go to the listen tab and check the listen to this device. one downside is that you will hear yourself too.
this was tested and works with analog headphone connections. i was testing with my usb headset, but had to go back to watching tv with the wife :P
if the usb headset could do this, the quality would be awesome. but what you say with the analog connections, there was a lot of extra environmental noise.
but looks like its possible without extra cables.
note: the post above is my opinion. as such, when reading any recommendations from me, please do you research and seek out other recommendations and make up your own mind on your next course of action. i mean, most reasonable adults should know that, but it seems this should be stated anyways.
You can't. A microphone port is an output port only.
I think you meant to say it's an input only, not an output. A speaker jack is an output.
OTOH, the OP's question is extremely confusing. Is he asking if he can play the output of his speakers as an input to his laptop via the microphone input? He never indicated which speakers he was referring to so it's possible he's talking about the speakers in his stereo system.
OP - what exactly are you trying to accomplish here?