WASAPI Loopback is the underlying technology I would use on Windows in order to get access to the main audio stream, although it will be getting everything that audio device is outputting rather than just the output from a single app.
So far I didnt find a trivial way to implement such things in UE4 so I have been inclined to fudge a bridge between these two worlds. eg have another app running that does the WASAPI Loopback stuff and frequency analysis, and then sends the results of that analysis, in the form of a small bunch of floats, to UE4 using Open Sound Control. This at least allows me to get going with prototyping and developing the visual side of things, and I can only hope a more direct method is available in future.