If you go to the windows Sound properties switch to the "recording" tab, and then right-click the blank area and ensure that "show disabled" and "show disconnected" are both enabled (checked), you should see an option for "stereo mix." Enable it, and then select that as your recording input for your streaming software.
"Stereo mix" is the aggregated audio output from all shared sources. Older versions of windows may have called it "what you hear" if I recall correctly.