Depending on what you want, this might be possible.
First, download the stand alone version of Stereo Tool.
Next, download one of the following two programs:
Virtual Audio Cable, if you need access for any audio source on your system. Not free. Very easy to set up.
I've used VAC for my first tests, so it should work without any problems with Stereo Tool.
Jack for Windows is free, but can only handle ASIO sounds. And it's a lot more difficult to use.
It will only work with the Stereo Tool 3.20 beta version (and I haven't tested it with Jack yet).
Next, set it up as follows. I will describe here how to do it with VAC, because most applications don't seem to support ASIO.
In the list of sound cards, "Virtual Audio Cable" will appear.
To route the sound from another program to Stereo Tool, configure the other program to play the output to "Virtual Audio Cable".
In Stereo Tool, set "Virtual Audio Cable" as the input sound card.
Now Stereo Tool receives and processes the data from the external program. Make sure to send the Stereo Tool output to a real sound card!