From what I have seen, the Aux Audio or maybe record what you hear option was provided by a driver not in Win 7. It was possibly the wdmaud.sys driver, but for whatever reason, possibly DRM, it is no longer available.
Maybe someone knows a way to install it, but I would suggest using a patch cord from your speaker to your line in. I have seen patch cords that have two connectors, so you can connect the front speakers and still have a plug for the line in plug.
I have not done it myself, but if I can find a patch cord, I will try. I am assuming when something is plugged into the line in, the graphic will show if you right click on the speaker icon and select recording devices.
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From my reading, it appears something called WaveMix was available up to the 7000 beta. In those, you could show disabled devices in the Playback or recording areas and the wavemix would show up. But it looks like it is no longer in the later versions. If anyone has a beta prior to 7100, maybe they could check to see if something might be done.