Hey dude do this little trick . right click on the speaker icon on the lower-right corner of taskbar & choose recording devices .
now right click on empty area of the list & check ( show disabled devices ) . now you will see Stereo mix . enable it . hit ok & go have fun
by the way make sure you have installed your sound driver ( realtek driver ) . other wise you cant active stereo mix .
if it didnt work . reinstall the game & try again so everything will be on its default . i had the same problem & this trick worked for me