Due to restrictions in OS X, it is not possible to monitor the system's audio output by default. Fortunately, developers have created audio routing utilities to make this possible. Soundflower is easily the most popular and the tool we've used most. If you need a hand getting it set-up, we've created a tutorial to get you going.
Now, you may be asking, "Why doesn't Spectre just include this utility?" Any of these routing utilites, such as Soundflower or Audio Hijack, require a kernel - a program that communicates directly with the hardware - to be installed separately from the application. If we were to provide our own kernel, it would literally be our own re-branded version of Soundflower that we would need to support separately from Spectre. We didn't see any need to re-invent the wheel, so we figured we would just send you straight to the source.
Otherwise, if your external audio interface has foldback/loopback capabilities, you can use that. Lastly, don't forget that Spectre supports AUNetSend/Receive, so if you're using an audio application that supports Audio Units, that's a great option as well.